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The
Health of Children and Young People
 5 Respiratory health
5.1 Introduction This chapter deals with self-reported symptoms and diagnosis of asthma. It also looks at lung function tests (age 7-24), and at their association with respiratory symptoms. A set of questions on respiratory health was introduced into the Health Survey in 1995.1 At that time, the questions were concerned with all household members aged 2 and over. From 2001 HSE included those aged zero and over, and data on those aged 0-24 are presented in this report. (For adults data see 2001 report.2) Asthma prevalence in children and young adults has increased in industrialised countries over the last few decades,3 and Britain has the highest rates of asthma in the world.4 An increase in the diagnosis of asthma by physicians may be due to changes in medical practice,5 but there is also evidence that symptoms such as wheezing are being reported more frequently by children and parents. Growing awareness of the condition has led to higher rates of diagnosis and treatment. In England, drugs for the respiratory system (chapter 3 of the British National Formulary) are the third most prescribed group of drugs, after those for the cardiovascular system and the central nervous system, both in terms of prescription items dispensed and net ingredient cost.6 In particular, drugs to treat asthma, such as corticosteroids and bronchodilators, are among the twenty most common drugs dispensed in the community, according to the most recent Prescription Cost Analysis (2002).6 Most hospital admissions due to the disease are emergencies: of the over 70,000 asthma episodes in England in the year 2001-02, 96% were admitted as an emergency according to Hospital Episode Statistics (HES),7 which show the toll to be especially high among young people: the mean age of those admitted to hospital was 28. Asthma is a disease of the respiratory system in which the airways are unusually sensitive to a wide range of stimuli. Exposure triggers a constriction of the airways, which is characteristically transient, and causes symptoms of tightnessof the chest and wheezing. The causes of this increase in sensitivity are still not clarified. A recent theory has linked the rise in atopic diseases (such as asthma and hay fever) to improvement in hygiene (the 'hygiene hypothesis'), which would cause insufficient exposure to certain infectious agents in early childhood, thereby preventing the immune system from being challenged early on, making it defective and favouring the development of allergic reactions.8 An increase in the circulation of potential irritants has been linked to the rise in the disease. Exposure to outdoor9 and indoor pollution and allergens10 has been investigated. The conclusion of a review by a sub-group of the Committee of the Medical Effects of Air Pollutants (COMEAP) was that, as regards the initiation of asthma, most of the available evidence does not support a causative role for outdoor air pollution.11 Other forms of air pollution, including indoor pollutants and irritants such as second-hand smoking, dampness and mould appear to be more relevant. Immunoglobulin E (IgE) has been identified as the antibody responsible for the immediate type of immune response:normal, nonallergic individuals have very low levels in their blood while in both allergic and parasitic disease its serum concentration may increase several-fold. Moreover, high initial levels in nonatopic subjects are predictive of the acquisition of atopy.12 Levels of specific IgE against house dust mites have also been associated with increased prevalence of symptoms: it is known that dust mites produce strong allergens that play an important role in the development of asthma and atopic conditions.13 Results on the distribution of serum IgE and house dust mite IgE and their association with symptoms are also reported in this chapter. Signs of impaired lung function can be useful in clinical settings for both the detection and the management of asthma. In the Health Survey sample the measurements are made for descriptive purposes, to assess the respiratory status of the population and to compare results in subgroups of the population in relation to respiratory symptoms and riskfactors for respiratory disease, such as smoking. 5.2 Methods The questions used in the Health Survey have been extensively used and validated in national and international studies. Infants from age 0 were included in the survey for the first time in 2001. For very small children, the reliability of reporting of symptoms by parents in epidemiological studies has been questioned.14 In particular, although the word 'wheezing' is extensively used in surveys of small children,15 other symptoms may be more relevant at this age, such as difficulty in breathing. For small children (aged 0-4), therefore, a question on difficulty in breathing was included in the HSE questionnaire alongside the question on wheezing or whistling. In a recent study, which tried to assess what parents understand by wheeze, 'difficulty in breathing' seemed to describe the symptoms better than 'wheezing'.14 Therefore for children aged 0-4 when 'wheezing' is mentioned in the report it refers to a positive answer to the question about 'wheezing' or 'difficulty in breathing' (see questionnaire, Appendix A, Volume 3). Measurement of lung function in young children is known to be difficult and less reproducible than in children of seven or more years of age. In this survey, as in previous HSE sweeps, only those aged seven and over were therefore considered eligible for the test. As in previous Health Surveys, three measures of lung function were used in this survey:
The Vitalograph 'Escort Spirometer', a portable spirometer with a calibration device, was used to measure lung function. This instrument measures the air expired by a subject using a flowhead containing a Fleisch pneumotachograph. The results are displayed on a small liquid crystal display. The manufacturer reports the accuracy of volume measurement as ± 3% or ± 0.05 litres, whichever is greater, and of flow measurement as ± 5%. These standards comply with ECSC (European Community for Steel and Coal) and ATS (American Thoracic Society, 1987) standards.16,17 The measurement technique involved a maximum inspiration followed by a forced expiration (for as long as possible) into the instrument. From each expiration, FEV1, FVC and PEF were calculated and displayed by the spirometer. The detailed measurement protocol is contained in Appendix B, Volume 3. Briefly, before visiting the first household on each working day, the nurse calibrated the spirometer by using a one-litre calibration syringe. Before measuring each informant, the ambient temperature was entered into the spirometer, which automatically corrects to BTPS conditions (body temperature and ambient pressure, saturated with water vapour at these conditions). The test procedures were demonstrated by the nurse using a detached mouthpiece. While in a standing position (unless chairbound), informants were asked to perform a forced inspiration, then an expiration with maximal effort and without hesitation. A test was considered as technically satisfactory if it was without the following: 1) an unsatisfactory start of expiration; 2) laughing or coughing, especially during the first second; 3) holding the breath in; 4) a leak in the system; 5) an obstructed mouthpiece. Lung function tests require the person being tested to learn the technique and to apply considerable effort to the task. In order to allow an opportunity to practise, and to allow also for invalid attempts, informants were asked to repeat the performance up to four more times, giving a maximum of five tests in total. The data presented here are based on the highest values for each of the three parameters (FEV1, FVC, PEF) from any of the technically satisfactory tests. The values used for a particular informant may not all come from the same test. It must be emphasised that data presented here should be interpreted with caution. Lung function measurements are affected by the way the informant performs the required task. This in turn is affected by the extent to which the technician is able to explain the task and to motivate informants to perform it as well as they possibly can. In spite of the careful development of protocols, extensive briefing and training of nurses, and the use of repetitions of the measurement to allow familiarisation, there remains a risk that readings may be lower in a home setting than under normal clinical conditions, where patients are likely to be even more strongly motivated to follow instructions than informants responding to a statistical survey. This may apply in particular to young children, who may have adapted to the technique less easily than others. However, there is no reason to believe that this possible underestimation varies between sub-groups of population, and internal comparisons, such as between regions, are still likely to be valid. In comparisons of these results with those from other studies, possible differences due to choice of equipment, measurement procedures and study settings should be taken into account. 5.3 Prevalence and severity of respiratory symptoms About a third of informants aged 0-24 reported a history of wheezing. Among children aged 0-15, the prevalence of ever wheezing was higher in boys (36%) than in girls (29%). Among young adults aged 16-24 the sex difference was less marked (young men 38%, young women 36%). About 55% of male informants and about 60% of female informants
who had ever wheezed reported wheezing in the last
twelve months (data not shown). Overall, about a fifth of informants reported
having wheezed in the last twelve months: 20% of boys aged 0-15 and 19%
of young men aged 16-24; and 17% of girls aged 0-15 and 21% of young women
aged 16-24.
Prevalence was higher
at both ends of the surveyed age range than in the intervening years.
Figure 5A shows, for each sex, the pattern of variation in prevalence
by age (moving averages of three age years). The chart also shows that
prevalence was higher among boys than girls, whereas among young adults,
in contrast, prevalence was higher among young women than young men. Table
5.1, Figure 5A The proportion who had ever had doctor-diagnosed asthma was 23% in boys and 18% in girls (aged 0-15); and 24% in young men and 23% in young women (16-24). It was higher in males than females at all ages; the difference between the sexes was relatively large among children 0-15, but the proportions converged in early adulthood, differences becoming small and non-significant. Prevalence was relatively low among very young children (7% in boys and 3% in girls aged 0-1) and then increased until the early teens. Prevalence then decreased slightly in males, but continued to increase among females. Figure 5B It is worth noting that of those who reported wheezing in the last twelve months, slightly more than half (59% of boys and 56% of girls; 56% of young men and 58% of young women) also reported a diagnosis of asthma (data not shown). More than two in five of those who had wheezed in the last twelve months did not have a diagnosis of asthma.
Most of
the informants with doctor-diagnosed asthma stated that the diagnosis
had been made more than twelve months earlier. 1% of young men reported
that the diagnosis had been made in the last twelve months, with 23% saying
longer ago (figures for young women were almost identical at 1% and 22%).
Of boys, 2% had had asthma diagnosed in the last twelve months, 21% longer
ago, while for girls the proportions were 2% and 16%. Table 5.1 Informants who reported wheezing attacks in the last twelve months were asked some questions to assess the severity of their symptoms, namely frequency of attacks, sleep disturbance and speech limitation (if applicable), and whether the symptoms had interfered with their daily activity. Examining the symptoms separately, 9% of boys aged 0-15 had had more than 12 attacks in the last twelve months, and prevalence was not dissimilar among girls (8%). The prevalence of frequent attacks was higher in those aged 16-24: 16% of young men and 18% of young women reported more than 12 attacks in the last twelve months. On the other hand, a higher proportion of informants aged 0-15 experienced nocturnal wheeze (20% in both sexes), and interference with their daily activities (56% of boys, 57% of girls) than of those aged 16-24: in this latter age group, these problems were reported more often by young women (15% and 50%) than by young men (9% and 39% respectively). The question about speech limitation was asked only from age 2 onwards: 17% of boys and 15% of girls aged 2-15 reported symptoms severe enough to limit speech to only one or two words between breaths, compared with 14% of young men and 22% of young women. Table 5.2 The relationship between these symptoms and the general health of informants was investigated by looking at how those with wheezing in the last twelve months perceived their health. Informants aged 13 and over were asked to rate their health in general on a five-point scale from 'very good' to 'very bad'. Those reporting their health as 'fair', 'bad', or 'very bad' were combined for the purpose of this analysis. In both sexes and all age groups, at least twice as high a proportion of those with wheezing in the last twelve months reported fair, bad or very bad health as of those who did not wheeze. Table 5.3 Wheezing symptoms appeared to be associated with psychological well-being as well as with general health. Informants aged 13-24 were asked to fill in a self-completion questionnaire. This included GHQ12, the General Health Questionnaire, based on 12 questions which ask about general level of happiness, depression, anxiety and sleep disturbance over the past four weeks. A score of 4 or more is used to identify possible psychiatric disorder. The proportion scoring 4 or more was higher among those with wheezing in the last twelve months than among their counterparts in the general population. This was true at all ages, and appeared to be more marked among those aged 20-24. In this age group 21% of young men and 30% of young women with wheezing scored 4 or more, compared with a population prevalence at this age of 13% (males) and 20% (females). Table 5.3 5.4 Prevalence of symptoms by indicators of socio-economic status 5.4.1 Socio-economic classification (NS-SEC) Wheezing prevalence in the last twelve months showed small differences by socio-economic classification, without consistent patterns in either sex. Among boys aged 0-5 the lowest prevalence was among those in households whose household reference person was in a lower supervisory or technical occupation (20%), the highest prevalence being found among boys whose household reference person was a small employer or own account worker (26%). In young men aged 16-24 the pattern was different: prevalence was 15% for intermediate occupations and 22% for lower supervisory and technical occupations. Females in households whose reference person was a small employer or own account worker tended to show lower rates than other groups. The prevalence of doctor-diagnosed asthma in children of both sexes was higher where the household reference person was in a semi-routine or routine occupation, while among young adults those in households with intermediate occupations had the highest prevalence, but the differences between other socio-economic groups were small. Table 5.4 5.4.2 Equivalised household income quintile Among both sexes and all age groups, wheezing prevalence tended to be higher in the lowest income quintile than in the highest quintile. The tendency for wheezing prevalence to decrease as income increased was less marked in boys aged 0-5. Figure 5C The table below shows a logistic regression model looking at the relationship
between equivalised household income and wheezing in the last twelve months.
The model controlled for age band and sex differences. The odds were significant,
indicating that income had an association with wheezing that was independent
of age and sex. For example, those in the highest income quintile were
about 20% less likely to have wheezed in the last twelve months than those
in the lowest quintile (odds ratio 0.80).
In boys and girls there was a clear trend in the prevalence of doctor-diagnosed asthma by equivalised household income: it decreased as income increased. Among those aged 16-24 the pattern was less clear, with smaller differences. Table 5.5, Figure 5D
 5.5 Geographical variation in the prevalence of symptoms 5.5.1 Government Office Region In spite of earlier studies reporting higher rates in the north of the country,18 no north-south (or east-west) geographical gradient was shown in the prevalence of asthma symptoms among school children aged 12-14 in England and Wales in ISAAC-UK.19 While the patterns observed in the present survey were not consistent in every region, there was an overall tendency for the northern regions to have higher symptom prevalence than southern regions. In the following table, the three northern regions (North East, North West, Yorkshire & the Humber) have been combined and compared with the three central (East Midlands, West Midlands, East England) and three southernmost regions (London, South East, South West).
For doctor-diagnosed asthma North East emerged as one of the regions with the highest rates, although this was not true for both sexes and all age groups. London and the South East on the other hand were generally more likely to have lower than countrywide prevalence. Grouping regions in the same way as for wheezing prevalence shows generally higher prevalence of doctor-diagnosed asthma in the north than the south of the country. Table 5.6
5.5.2 Area deprivation (IMD, Index of Multiple Deprivation) The Index of Multiple Deprivation (IMD) ranks areas from most deprived to least deprived. This classification is based upon area characteristics in six domains: income, employment, health deprivation and disability, education, housing, and access to services.20 The analysis is based on IMD quintiles. The association between wheezing and relative area deprivation is shown in Table 5.7. Those living in the most deprived areas tended to have a higher prevalence of wheezing than those living in less deprived areas, but no clear trend emerged. The table shows two logistic regression models looking at the relationship between area deprivation and the prevalence of wheezing in the last twelve months. The first model controls only for age band and sex differencesbetween areas, the second for age band, sex and equivalised household income. While the odds of wheezing varied to some degree between area deprivation quintiles, there is no conclusive evidence of an association between area deprivation and wheezing prevalence. In Model 1, there was a borderline significant difference between the most deprived and least deprived quintiles, but not in Model 2, which controlled for equivalised household income as well as for age and sex. Table 5.7, Figures 5E, 5F
5.5.3 Degree of urbanisation The degree of urbanisation of the household's area can be considered as a very general indicator of outdoor pollution. In the Health Survey the classification of areas as urban or rural is based on the interviewer's assessment at the address. For both sexes and both age groups there were no clear differences in the prevalence of wheezing in the last twelve months between those who lived in urban and those who lived in rural areas. There was also no significant tendency for doctor-diagnosed asthma to be more common in those who lived in urban areas. Table 5.8 In summary, from the analyses of these data, the association between wheezing (or doctor-diagnosed asthma) and the various indicators of socio-demographic status does not appear to be very strong. There was a tendency for wheezing to be reported more by less privileged groups, equivalised household income being the indicator that showed the strongest association: the prevalence of wheezing and doctor-diagnosed asthma increased as income decreased. 5.6 Exposure to indoor risk factors 5.6.1 Indoor risk factors The factors considered to be a potential risk for wheezing and asthma in the confined environment (indoor risk factors) are shown in Tables 5.9 and 5.10. In the literature, they have been associated in various degrees with the risk of asthma.21 The use of domestic gas appliances and keeping household pets did not show a significant association with wheezing in the last twelve months or with doctor-diagnosed asthma, in either sex. The presence of condensation and mould or damp (reported, not observed) showed a positive association with wheezing which was statistically significant among females. For example, 19% of females aged 0-15 and 24% of those aged 16-24 with signs of condensation in the household reported wheezing, compared with 14% and 18% respectively of those whose household was without such signs. These results broadly confirm what has already been reported in HSE 1997.27 A risk factor reputedly important for wheezing and asthma is exposure to other people's smoke (second-hand smoking). Informants were classified into two groups, 'more exposed' and 'less exposed'. Those more exposed comprised informants aged 2-12 who were looked after for at least two hours a week by someone who smoked during that period, and informants aged 13-24 who reported exposure of at least 6 hours a week to others' smoke. The prevalence of wheezing in the last twelve months was significantly associated with exposure in both age groups and both sexes. This analysis included smokers. While only a small proportion of younger children smoked, about a third of young adults were smokers (see Chapter 1, Sections 1.2.1 and 1.3.1). The analysis of those aged 16-24 was therefore repeated excluding those who smoked. The differences between those exposed and unexposed to second-hand smoke were still appreciable but reduced in both sexes, and still significant in females, as the table below shows: wheezing prevalence was 15% in exposed and 13% in unexposed males; 21% in exposed and 15% in unexposed females. Table 5.9
The prevalence of doctor-diagnosed asthma in boys aged 0-15 was significantly higher in those exposed to second-hand smoke than in those not exposed (28% vs. 21%) but not in young men aged 16-24, where the proportions were 26% and 22% respectively. In females a pattern similar to that for males emerged: the prevalence of doctor-diagnosed asthma was 22% in exposed girls and 16% in unexposed girls, and 24% and 23% respectively in young women aged 16-24. The exclusion of smokers from those aged 16-24 did not change the results: Table 5.10
5.6.2 Own cigarette smoking habits Information on smoking behaviour was available for informants aged 18-24, who gave their answer directly to the interviewer, and for those aged 8-17, collected by self-administered questionnaire; the questions asked of children aged 8-15 differed from those asked of informants aged 16-24 (see Chapter 1, Section 1.1.2). Caution is needed in interpreting the results for children aged 8-15, since there is reason to believe that the Health Surveys under-report smoking prevalence in this age group (also see Chapter 1, Sections 1.2.3, 1.4.2). Among young adults aged 16-24, wheezing in the last twelve
months was more prevalent in cigarette smokers than in those who had never
smoked or in ex-smokers, and increased with the amount smoked. While 14%
of young men and 17% of young women who had never smoked reported wheezing,
37% of young men and 39% of young women who smoked 20 or more cigarettes
a day reported wheezing. Table 5.11, Figure 5G
Among those aged 8-15
the answer to the self-administered questionnaire was adjusted to take
into account the cotinine level of the child. Cotinine, a derivative of
nicotine, was measured in the saliva of informants (see Chapter 1, Section
1.4). It is used as an objective marker of smoking: a value of 15 ng/ml
or more can be taken as indicative of smoking. A number of children stated
they were not smokers/were infrequent smokers/used to smoke but were not
current smokers, but had cotinine levels indicative of smoking. These
cases can be reclassified as current smokers ('smoke one or more a week')
for the purpose of assessing whether the relationship between wheezing
symptoms and smoking behaviour is affected by the known propensity of
some children to under-report smoking. After this adjustment for cotinine wa s made, in this age group, as observed among informants aged 16-24, the prevalence of wheezing was higher in those who smoked (and/or had high cotinine levels) than in those who did not among boys, while the differences were not significant in girls. Table 5.12, Figure 5H
5.7 Familial characteristics (children 0-15) The analysis of wheezing symptoms by familial characteristics was carried out only in children aged 0-15, and this section refers only to this age group. While in 2001 the questions about respiratory symptoms and asthma were asked of all informants, in 2002 these questions were only asked of informants aged 0-24. Hence the parents of children included in the boosted sample were not asked the respiratory questions except where they themselves happened to be under 25 years of age (see also Volume III, Methodology & Documentation). The associations between wheezing and parental wheezing symptoms and asthma could therefore be investigated only for the year 2001. In general, symptoms were related to the size of the household. The prevalence of wheezing was highest in two person households (a child living with only one adult) (29% in boys aged 0-5, 24% in boys aged 6-15; and 27% and 23% in girls respectively), and decreased in larger households: those with five or more people (typically father, mother, three or more children) had the lowest prevalence of wheezing: 21% in boys aged 0-5, 17% in those aged 6-15; and 19% and 14% in girls respectively. Table 5.13 Paternal smoking did not seem to be related to children's wheezing symptoms, but maternal smoking was: boys and girls whose mothers currently smoked cigarettes were more likely to have wheezed than those whose mothers had never regularly smoked cigarettes. In boys aged 0-5, 29% of those whose mothers currently smoked cigarettes reported wheezing, compared with 23% of those whose mothers had never regularly smoked. The corresponding figures for boys aged 6-15 were 20% and 16%. Among girls aged 0-5, 22% of those whose mothers currently smoked cigarettes reported wheezing, compared with 16% of those whose mothers never regularly smoked. Among girls aged 6-15 the corresponding figures were 18% and 13%. Table 5.14 Paternal and maternal wheezing symptoms were related to boys' and girls' wheezing. Wheezing was reported by 29% of boys aged 0-5 and 23% of boys aged 6-15 whose fathers wheezed in the last twelve months; and by 19% and 13% respectively of boys whose fathers did not report wheezing. Among girls the corresponding figures were 25% and 21% where the father wheezed, and 16% and 11% where the father did not wheeze. The association with maternal wheezing was even stronger, especially in the younger age group: 38% of boys and 30% of girls aged 0-5 whose mothers wheezed in the last twelve months reported wheezing, compared with 18% of boys and 15% of girls aged 0-5 whose mothers did not wheeze. Table 5.15 Wheezing symptoms were also more common in children whose fathers or mothers reported doctor-diagnosed asthma. Wheezing prevalence was 39% in boys aged 0-5 and 26% in boys aged 6-15 whose fathers had doctor-diagnosed asthma; and 17% and 13% respectively in boys whose fathers did not have asthma. Among girls the corresponding figures were 20% and 23% of those whose fathers had asthma, and 17% and 12% of those whose fathers did not have asthma. Wheezing symptoms were similarly reported more often by girls whose mothers had doctor-diagnosed asthma than those whose mothers did not have asthma. Table 5.16 5.8 Logistic regression for wheezing in the last twelve months 5.8.1 Prevalence of wheezing symptoms by risk factors in children aged 0-15 The relative odds of wheezing in the last twelve months were examined using logistic regression analysis with a variety of independent predictor variables: exposure to other people's smoke (less than two hours, two or more hours a week for those aged 0-12, exposure of less than 6 hours, 6 or more hours a week to others' smoke for those aged 13-15), maternal and paternal doctor-diagnosed asthma, maternal smoking habits (in four categories: never regularly smoked, ex-regular smoker, currently smokes under 20 cigarettes a day, currently smokes 20 or more cigarettes a day), household size (2, 3, 4, 5 and over), degree of urbanisation (urban, suburban, rural), equivalised household income (in quintiles), reported presence of condensation or mould in the house. Age was entered as a categorical variable (0-2, 3-6, 7-12, 13-15). Separate models were run for boys and girls, and separately for two-parent households and lone-parent households (with single mothers). These analyses were carried out on the 2001 data only (see Section 5.7). It is interesting to note that half (51%) of children in lone parent households belonged to families in the lowest income quintile and only 3% of them were from families in the highest income quintile (data not shown). Two-parent households Being exposed to other people's smoke was not associated with increased odds of wheezing, and neither was maternal smoking. Both maternal and paternal doctor-diagnosed asthma were associated with increased odds of reporting wheeze in the last twelve months in boys. Boys whose fathers had asthma were three times as likely to report wheezing as those whose fathers did not have asthma (odds ratio 3.09), and those with an asthmatic mother were almost twice as likely to wheeze as those whose mother was not asthmatic (odds ratio 1.92). The corresponding odds in girls were significant only for maternal asthma: those with an asthmatic mother were more than twice as likely to report wheezing than those whose mothers did not have asthma (odds ratio 2.35). In boys, there was no association between household size and wheezing while in girls those living in large families appeared to be more likely to report asthma (odds ratio 1.72). There was no association between urban or rural residence and wheezing in boys, while in girls those living in rural areas were half as likely to report symptoms as those living in urban areas (odds ratio 0.55). After adjustment for the other factors, equivalised household income did not appear to be associated with wheezing. In both sexes presence of condensation or mould in the house was not associated with an increased risk of wheezing. Table 5.17 Lone-parent households In boys, maternal diagnosis of asthma was likely to be associated with increased odds of wheezing (odds ratio 1.69) although it failed to reach statistical significance, due to the relatively small sample analysed. In girls maternal asthma was significantly associated with
increased odds of wheezing (odds ratio 2.17). Table 5.18
5.8.2 Prevalence of wheezing symptoms by risk factors in those aged 16-24 Among young adults the risk factors considered in the logistic regression analyses were own smoking status (never regularly smoked cigarettes, ex-regular smoker, smokes under 20 cigarettes a day, smokes 20 or more cigarettes a day), exposure to other people's smoke (less than six hours, six hours or more a week), degree of urbanisation (urban, suburban, rural) and equivalised household income (in quintiles). In young men, own smoking status emerged as the most important risk factor, after adjusting for the other factors. Smoking under 20 cigarettes a day was associated with a doubling of the risk of wheezing (odds ratio 1.90) compared with those who had never smoked (or never smoked regularly), while those who smoked 20 or more cigarettes a day were more than three times as likely to wheeze (odds ratio 3.21). Among young women own smoking status was significantly associated with increased risk of wheezing: those who smoked 20 or more cigarettes a day were two and half times more likely to wheeze than those who did not smoke (odds ratio 2.44). Among young women household income showed an association with wheezing. Compared with those in the lowest income quintile, those in higher income quintiles had lower odds of wheezing. Exposure to other people's smoke also showed a significant association with wheezing in young women. This association was also seen in young men, but not at a significant level. Table 5.19 Since the inclusion of equivalised household income reduced the sample size considerably, the analysis was repeated excluding this variable from the logistic regression. A similar association between wheezing symptoms and the three variables mentioned above emerged (data not shown). 5.9 Trends in respiratory symptoms and asthma Questions on wheezing and asthma were the same in 2001 and 2002 as in 1995-1997, but while HSE 2001 and 2002 collected information on the whole age range, HSE 1995-1997 collected data for those aged 2 and over only. The present comparison with HSE 1995-1997 therefore refers to the age group 2-24 only. The proportion ever having wheezed increased between 1995-1997 and 2001-2002, in both sexes and in all age groups, as Table 5.20, Figure 5J and the table below show. The change is shown as percentage points (pp) in the third and sixth columns.
On the other hand the prevalence of wheezing in the last twelve months remained stable over the same period. This apparent discrepancy between trends in 'lifetime' and 'recent' wheezing could be explained by assuming that a better control of wheezing symptoms in children and young adults has been achieved in recent years, so that those with the disease experienced relatively fewer symptoms in the last twelve months even though the tendency was for the disease to increase over time. Table 5.20, Figures 5I, 5J
5.10 Serum IgE and house
dust mite specific IgE
5.10.1 Measurement
IgE and house dust mite specific IgE were measured in the
blood samples of those aged 11 and over. Table 5.21 presents the response
rates to the IgE sample. Valid samples were obtained from 59% of males
and 50% of females who had a nurse visit. The response rates were lower
in those aged 11-15 (50% boys, 45% girls) than in those aged 16-24 (67%
and 54% respectively). Given that in both age groups the refusal rates
were high, some caution is needed when interpreting the results, as the
informants from whom the blood sample was obtained may not be entirely
representative of the whole group of persons aged 11-24. Those with valid
sample were significantly older than all those eligible (mean ages 17.3
and 16.7 respectively). However, when comparing those from whom the sample
was obtained with all those eligible to give blood in this age group,
no significant differences emerged in income distribution, degree of urbanisation,
or wheezing symptoms between the two groups.
The distribution of IgE was very skewed and the geometric
mean was therefore calculated as well as the arithmetic mean. The geometric
mean is a measure of central tendency. It is sometimes preferable to the
arithmetic mean, since it takes account of positive skewness in a distribution.
The geometric mean of a continuous variable is calculated by taking the
antilog of the mean of the logged original values. Table 5.21
5.10.2 Serum
IgE
Serum IgE values were higher in males than in females,
mainly due to differences in those aged 16-24. Serum IgE values were higher
in females aged 11-15 than those aged 16-24, while in males there was
no age difference. Table 5.22
The geometric mean of total IgE is presented in Table 5.23
for those who had wheezed in the last twelve months and those who had
not. The IgE geometric means were higher for those who reported wheezing
in the last twelve months than for those without the symptom. This was
true in both sexes and age groups. Table 5.23
5.10.3 House dust mite specific
IgE
A house dust mite value of 0.4 kU/l is considered to be
the reference threshold for increased risk of allergic reactions. The
percentages of males and females with a valid house dust mite IgE sample
that exceeded this 0.4 kU/l threshold are shown in Table 5.22. About a
third of male informants and a quarter of females had a house dust mite
IgE sample that exceeded 0.4 kU/l. Table 5.22
The percentages of males and females with a valid house
dust mite IgE sample that exceeded the 0.4 kU/l threshold are shown in
Table 5.23 for those who had wheezed in the last twelve months and those
who had not. About half of males with wheezing symptoms (53% of those
aged 11-15 and 50% of those aged 16-24) and more than a third of females
(36% and 34% respectively) had a house dust mite IgE in excess of the
reference value of 0.4 kU/l. Table 5.23
5.11 Lung function
5.11.1 Response rate
Among those who had a nurse visit, valid lung function
measurements were provided by 98% of boys and of girls aged 7-15, and
by 99% of young men and 94% of young women aged 16-24. Refusal rates were
very low (around 2%), and the slightly lower response rate among young
women is due to the exclusion of pregnant women (4%). Table 5.24
Among those with valid measurements 90% were of white origin,
5% of Asian background, 2% considered themselves to be of black origin
and 2% were of mixed ethnic group (data not shown). It has long been recognised
that race differences in lung function exist.24
Given the small number of informants of minority ethnic origin, and for
consistency with previous reports, the results presented here include
all ethnic groups. In some instances, when the results observed may partly
be explained by the ethnic origin of the informants, the analysis is also
presented for white informants only (see Section 5.11.4, Government Office
Region).
5.11.2 Lung function by age and sex
FEV1
Mean FEV1 was higher in males than in females,
with differences increasing with age. In both sexes marked differences
were observed between the shortest and the tallest height tertiles, with
differences being bigger than between two consecutive age years in the
same height tertile.
In both sexes FEV1 increased markedly with age
in the younger ages, up to the mid teens. From then on it only showed
small increases, reaching a peak in the early twenties. Table 5.25, Figure 5L
FVC
The distribution of FVC was similar to that of FEV1,
with rates increasing steeply up to teenage years in both sexes and levelling
off in the early twenties. Table 5.26, Figure 5M
PEF
The distribution of PEF was also similar to that of FEV1
and FVC. Table 5.27, Figure 5N
5.11.3 FEV1
by indicators of socioeconomic status
Socio-economic classification
(NS-SEC)
Among boys aged 7-9 and 10-12, height-adjusted mean FEV1
was lower than average in those where the household reference person was
in an intermediate occupation, and this was also true for young men aged
20-24.
Among girls aged 7 to 15 those in intermediate households
also tended to have lower FEV1 than the average, while this
did not apply to young women aged 16-24. Table 5.28
Equivalised household
income
Height-adjusted differences between quintiles of income
were small in males, and no clear pattern emerged.
Among females mean FEV1 tended to increase with
the level of income, in most age groups: those in the top income quintile
had higher mean FEV1 than those in the bottom quintile of income. Table
5.29
5.11.4 Geographical variation in mean
FEV1
Government Office Region
In males of all ages, height adjusted mean FEV1
was higher than average in Yorkshire and the Humber and lower than average
in London. London also showed lower than average mean FEV1
in females. Table 5.30
Given that people from minority ethnic groups tend to have
lower lung function results,25 the higher than average concentration
of people from minority ethnic groups in London could partly explain these
results. When the analysis by region was repeated restricting the observations
to white informants only, the mean FEV1 levels of children
and young adults living in London were for most age groups closer to the
country-wide average, as the table below illustrates.
Country average
The results for the other regions were only marginally affected
by the exclusion of non-white informants (data not shown).
Area deprivation (IMD,
Index of Multiple Deprivation)
The index of area deprivation used was the Index of Multiple
Deprivation20 (IMD), areas being divided into quintiles on
the basis of this. There was no clear trend in height adjusted mean FEV1
by IMD quintile, except for a tendency in both sexes for it to be slightly
lower in the most deprived than the least deprived quintile. The middle
three quintiles did not show any consistent trend. Table 5.31
5.11.5 FEV1
by respiratory symptoms and smoking habits
In males, those who reported wheezing symptoms in the last
twelve months were more likely to have lower mean FEV1 than
those without such symptoms, and the differences tended to be more marked
among older children and young adults than among younger children. Among
females a similar pattern emerged, though there was no significant difference
among girls aged 7-9. Table 5.32
Those with doctor-diagnosed asthma tended to have lower
mean FEV1 than those who had not been diagnosed as asthmatic.
This was true for both sexes and all age groups. Table 5.33
Table 5.34 shows the association between FEV1
and smoking for those aged 10 and over only. There were too few smokers
among younger respondents for the data to be presented. In both sexes
there were no differences in mean FEV1 by smoking status. Table
5.34
To quantify the association of respiratory symptoms and
smoking status with FEV1 in those aged 16-24, adjusting for
age and height, a linear regression was performed. The variables corresponding
to the questions on wheezing in the last twelve months and doctor-diagnosed
asthma were included in the analysis, as well as own smoking status grouped
as: never regularly smoked, ex-regular smoker, smokes under 20 cigarettes
a day, smokes 20 or more cigarettes a day.
In males, subjects with respiratory symptoms in the last
twelve months had significantly lower FEV1 than subjects without
symptoms, though the differences were small. Doctor-diagnosed asthma and
smoking did not show a significant association with a lower FEV1.
Females who reported wheezing in the last twelve months
had mean FEV1 0.07 litres lower than those without symptoms,
and those with doctor-diagnosed asthma had FEV1 0.08 litres
lower than those without the diagnosis, while no significant association
with smoking was seen. When interpreting these results,
it is worth considering that the measurements, even in asthma sufferers,
were taken while the informant was in a relatively relaxed condition,
not under undue stress and certainly not under an attack. Given the reversibility
of the air flow limitation in asthma, this could account for the smallness
of the reduction observed in mean FEV1. Table 5.35
5.11.6 Changes in mean FEV1
between 1995-1997 and 2001-2002
Table 5.36 and Figure 5O compare the FEV1 age
and sex pattern between 1995-1997 and 2001-2002. The figures for the two
sets of years were almost the same in both sexes. Table
5.36, Figure 5O
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