Prevalence of wheezing in the chest among adults from the 1982 Pelotas birth cohort , Southern Brazil

MÉTODOS: Estudo prospectivo de coorte dos nascidos em 1982 na cidade de Pelotas (RS). Foram localizados 4.297 (77,4%) dos membros da coorte em 2004-5, cujos dados foram coletados por meio de entrevista, utilizando o questionário ISAAC (International Study of Asthma and Allergies in Childhood Steering Committee). A associação entre o desfecho “ocorrência de chiado no peito nos 12 meses anteriores à entrevista” e variáveis socioeconômicas, demográficas e características ao nascimento foi calculada por análise multivariável utilizando regressão de Poisson.


INTRODUCTION
The prevalence, incidence and severity of asthma are increasing worldwide.Although the number of studies on asthma in childhood is increasing, 18 most of the data on the prevalence of asthma among young adults comes from developed countries. 5Studies in middle or low-income countries are needed for better understanding of the epidemiology of this disease among adults.
The various diagnostic methods used in clinical practice are still little used for epidemiological surveys, particularly for household-based surveys.In most such studies, the diagnosis is based on reported symptoms, especially wheezing in the chest. 18Population-based studies may also help in understanding the role of socioeconomic factors and early infl uences on the epidemiology of asthma.
The aims of the present study were to estimate the prevalence of wheezing in the chest among young adults and to explore the effect of some independent variables on occurrences of this morbidity.

METHODS
All the births that occurred in hospitals in the city of Pelotas in 1982 were identifi ed.The mothers were interviewed and the newborns were weighed.This population was followed up on different occasions.In 2004-5, all the members of the cohort were sought, and a monitoring rate of 77.4% (N = 4,297) was achieved.Details of the methodology of the cohort have already been published. 4,15,16 defi ne the outcome of the present study, the questionnaire of the International Study of Asthma and Allergies in Childhood Steering Committee (ISAAC) was used. 3,18This has already been validated in Brazil. 1 Firstly, the subjects were asked whether they had ever had wheezing in the chest during their lives.Then, those who answered affi rmatively were asked how many crises of wheezing in the chest they had had over the past year and whether they had any family history of paternal or maternal asthma.
The independent variables gathered were: sex; selfreported skin color (White or Black/Mixed); family income reported by the mother in 1982, in minimum monthly wages (MMW); family income in 2004-5 (in MMW); birth weight, in grams; and duration of breastfeeding, in months.Based on the income at birth and present income, the change in income between 1982 and 2004-5 was defi ned and categorized into four groups: always poor; non-poor to poor; poor to non-poor; and never poor.The individuals within the lowest tercile of income were defi ned as poor.
Bivariate analyses were based on the chi-square test for heterogeneity or linear trend.Poisson's regression was used for multivariable analysis stratifi ed by sex and in accordance with a conceptual model in which the variables of skin color, family history of asthma, maternal schooling level and family income at birth were grouped into the fi rst level.The variables of birth weight and duration of breastfeeding were analyzed separately, in the second and third levels, respectively.For the adjusted analysis, all the variables with p < 0.20 were kept in the model.
Verbal informed consent was obtained from the adults responsible for the children during the phase of the study from 1982 to 1986, as was the common practice at that time, when there was no ethics committee at the Federal University of Pelotas.For the more recent phases, the university's ethics committee, which is affi liated to the National Council for Research Ethics (Conselho Nacional de Ética em Pesquisa, CONEP), approved the study and written informed consent was obtained from the participants.

RESULTS
Among the young adults who answered the questionnaire on asthma, 2,231 (52.0%) reported that they had had wheezing in the chest at some time during their lives.A total of 1,067 members of the cohort (24.9%) reported wheezing in the chest over the past year.The mean number of crises of wheezing reported by these individuals over the 12 months preceding the interview was 5.7, with a median of two crises.Among those who reported wheezing in the chest over the past year, 54.6% said they had diffi culty in sleeping and 12.9% had diffi culty in speaking, consequent to the crises.The prevalence of dry coughing at night without colds, over the 12 months preceding the interview, was 38.7%, while 14.4% of the interviewees reported wheezing in the chest after doing physical exercise.Family histories of asthma (father or mother) were reported by 8.8% of the interviewees.
Table 1 presents the prevalence of wheezing in the chest over the 12 months preceding the interview, according to the independent variables and stratifi ed by sex.For both sexes, the prevalence was greater among individuals with a family history of asthma.Among the women, mixed or Black skin color and low socioeconomic level were associated with greater prevalence of wheezing.Among the men, those who had never been poor presented lower frequency of this outcome.For both sexes, birth weight and duration of breastfeeding did not present associations with occurrences of wheezing in the chest.
Table 2 shows the prevalence ratios in the crude and adjusted analyses for the men.Individuals with a family history of asthma presented a risk of wheezing in the chest that was around twice the risk among those without a family history of this condition.Individuals who had never been poor presented lower risk.Even after adjusting for possible confounding factors, skin color, birth weight and duration of breastfeeding were not associated with a risk of wheezing in the chest.Table 3 presents the results for the women.Skin color and family history of asthma continued to be associated with wheezing after adjustments.Low family income at birth and poverty throughout life were associated with this outcome, and also in the adjusted analysis.The absence of effects from the variables of birth weight and duration of breastfeeding on the prevalence of wheezing in the chest among the women persisted in the adjusted analysis.

DISCUSSION
Around a quarter of the young adults in the 1982 Pelotas cohort reported wheezing in the chest over the past year.In the National Health and Nutrition Examination Survey (NHANES III), conducted in the United States, the prevalence of wheezing in the chest over the 12 months preceding the interview, among adults aged 20 years and over was 16.4%, while the prevalence of a medical diagnosis of asthma was 4.5%. 2 In the Behavioral Risk Factor Surveillance System of 2000 (BRFSS-2000), 7.2% of the adults living in the United States reported that a doctor had told them that they had asthma and that the symptoms continued until the time of the interview. 8In Australia, the prevalence of wheezing in the chest over the last year ranged from 17% to 29% among adults (20 to 44 years of age). 17In a European study covering several countries, the prevalence of asthma among young adults (20 to 44 years of age) was 4.5%, although there was great variability between the countries. 9The study by Pearce et al 13 showed that there was high concordance between the instruments used in ISAAC and the European Committee of Respiratory Health Survey, 13 which allows comparison between our fi ndings and those from the European study.
In the present study, the women presented a risk of reporting wheezing in the chest that was 12% greater than the risk among the men.This corroborates the data from BRFSS-2000, NHANES III for wheezing in the chest 2 and another Brazilian study on asthma symptoms. 11However, it needs to be borne in mind that, in childhood, the prevalence of wheezing in the chest is greater among boys than among girls. 6In our study, the Black or Mixed women presented a greater risk of wheezing in the chest, thus confi rming the data of BRFSS-2000, in which Blacks presented greater occurrence of asthma than did Whites. 8On the other hand, in NHANES III, the prevalence of wheezing in the chest over the past year was slightly higher among Whites than among Blacks. 2 Thus, there is no agreement between the studies regarding this association.
Our results showed that individuals with lower income presented a higher risk of wheezing in the chest, which was also in agreement with data from BRFSS-2000. 8 A study conducted in Pelotas on adults aged 20 to 69 years showed that those with lower family income presented a higher risk of asthma symptoms than did those with higher family income, both in the crude analysis and in the adjusted analysis. 11In another cohort study carried out in Pelotas, the prevalence of wheezing in the chest was greater among the young people of low socioeconomic level, both during childhood and at the start of adolescence. 6,12In a previous follow-up on this cohort 10 that was conducted at the time of the military call-up for the young men, the frequency of wheezing in the chest was greater among those of high family income.These data are concordant with the "hypothesis of hygiene", which proposes that infec- tions during childhood may provide protection against asthma during adulthood.In the present analysis, which was conducted among individuals of both sexes fi ve years after the latter, the association between family income and wheezing in the chest was in the opposite direction.In the earlier paper, a hypothesis of information bias was raised, given that reports on asthma could be different between young people of high and low socioeconomic level. 10By cross-referencing the reports from the two interviews, among individuals who said at the military interview that they had had wheezing in the chest during the past year, the following percentages reported wheezing in 2004-5: 55.4% of the lowest tercile of income, 45.4% of the middle tercile and 42.7% of the highest tercile.This suggests that some young men with high income erroneously said that they had wheezing at the time of the military call-up, possibly to avoid recruitment.This bias may explain the discrepancy between the results.
The effect of changes of income on the frequency of wheezing in the chest showed that individuals exposed to poverty, whether during childhood or during adulthood, presented greater risk.It was not possible to detect whether exposure to poverty during childhood was more harmful than exposure during adulthood, or vice versa.
The study among the recruits also showed that prolonged breastfeeding increased the risk of wheezing in the chest, 10 which was not confi rmed in the present analysis.On the other hand, recent meta-analyses have shown that breastfeeding has a protective effect or no effect on occurrences of asthma. 7,14 was decided to present the analysis with stratifi cation by sex, given that a recent paper showed that the risk factors for wheezing in the chest among adolescents aged 10 to 12 years were very different between boys and girls. 12However, among young adults, little difference in the risk factors for wheezing was detected between men and women.
Some limitations of the present study must be taken into consideration.The diagnosis of asthma was based on symptoms of wheezing in the chest, which although being a good indicator for occurrences of asthma, do not constitute a confi rmed diagnosis.However, there is no gold-standard method for diagnosing this disease.The symptom of wheezing in the chest is used internationally and has been shown to be the best option for epidemiological studies.Furthermore, although a recall period of 12 months was used, as recommended in the literature, there is the possibility of some degree of memory bias.

Table 1 .
Prevalence of wheezing in the chest, according to sex.Pelotas, Southern Brazil, 1982 to 2004-5.

Table 2 .
Crude and adjusted analyses on the effects of the independent variables on the prevalence of wheezing in the chest over the 12 months preceding the interview, for males.Pelotas, Southern Brazil, 1982 to 2004-5.Adjusted for skin color, family history of asthma, maternal schooling level and family income at birth, at the fi rst level, birth weight at the second level and duration of breastfeeding at the third level.
** Wald test for heterogeneity *** Wald test for linear trend

Table 3 .
Crude and adjusted analyses on the effects of the independent variables on the prevalence of wheezing in the chest over the 12 months preceding the interview, for females.Pelotas, Southern Brazil, 1982 to 2004-5.