From the Department of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA.
Received for publication June 7, 2002; accepted for publication October 14, 2002.
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ABSTRACT |
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absenteeism; child; respiratory system; respiratory tract diseases; schools; tobacco smoke pollution
Abbreviations: Abbreviations: CI, confidence interval; ETS, environmental tobacco smoke; RR, relative risk.
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INTRODUCTION |
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A substantial body of evidence indicates that exposure to environmental tobacco smoke (ETS) contributes to the occurrence of respiratory symptoms and diseases in infants and younger children (13). Adverse respiratory health outcomes associated with ETS include increased occurrence and severity of symptoms, transient changes in lung function, increased numbers of respiratory infections, more visits to physicians and emergency rooms, and increased hospital admissions. Because illness-related absences from school are common events that represent a broad spectrum of morbidity from mild, transient illnesses to the most severe illnesses requiring emergency room visits or hospital admissions, consideration of school absenteeism may provide a useful integrative assessment of the adverse impact of ETS exposure during childhood (4).
Although most absences are associated with illnesses at the low end of the morbidity spectrum, an absence indicates an illness of sufficient severity to affect the childs daily functioning as well as child and family coping strategies (47). Repeated absenteeism is also associated with lower academic performance and poor social adjustment (4, 7, 8). Illness-related absences may also have negative economic effects on families if a parent misses work to care for a sick child. Although illness-related school absenteeism is an important adverse outcome, few studies have investigated the effects of ETS on illness-related absenteeism in children, a group identified as especially sensitive to the adverse effects of tobacco smoke and other air pollutants (9, 10).
The Childrens Health Study offers an opportunity to investigate the effects of ETS on respiratory-illness-related absences (11, 12). We conducted a substudy within this study cohort, the Air Pollution and Absence Study, and examined data on the incidence of type-specific absences collected by using an active surveillance system in a cohort of fourth-grade schoolchildren aged 812 years who attended schools in the 12 Childrens Health Study communities during JanuaryJune 1996 (13).
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MATERIALS AND METHODS |
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Participant characteristics
Sociodemographic information, indoor exposures, and medical histories were obtained from questionnaires completed by parents or guardians at study entry in the fall of 1995. The subset of participants with asthma was defined by using parent-reported history of physician-diagnosed asthma. Children with wheezing were defined as those with a history of wheezing in the 12 months prior to study entry. Parental education was categorized by using the highest level for the parent or guardian who completed the questionnaire. Information regarding number of hours spent outdoors over a 1-week period was collected by a self-administered questionnaire and was categorized as "more outdoors" or "less outdoors" on the basis of whether the children spent more than or less than the median number of hours (11.25 hours) outside. Weight and height were measured by using standard protocols during interviews at school. Body mass index was categorized into sex-specific quartiles.
Tobacco smoke exposure information was collected by using questionnaire items about the current and past household smoking status of each participants mother, father, other adult household members, and regular household visitors. The current number of household smokers (1, 2, 3, 4, 5, 6 or more) was recorded. Based on the distribution of responses, the number of smokers was categorized as 0, 1, and 2 or more household smokers. Personal smoking habits on the previous day, week, and month were assessed during a private interview with each child.
Absence surveillance
We collected school absence reports from 27 elementary schools attended by the newly recruited fourth-grade children for the period January 1, 1996, to June 30, 1996. Of the 2,081 children in the fourth-grade group, 2,068 were eligible for absence surveillance because they were enrolled in the Childrens Health Study at the beginning of the surveillance period. Of these 2,068 children, 135 were excluded from the analysis for the following reasons: 32 withdrew from the study, 90 changed schools during the study period, and 13 did not have absence data because of administrative errors.
Daily absence information was collected by using methods described previously (12). Briefly, attendance reports were requested from schools every 24 weeks, with the interval depending on the availability of personnel and electronic data systems at individual schools. An absence was defined as a day or an adjacent series of school days on which a participant did not attend school when it was in session. Over the study period, we ascertained 5,665 absences.
We established an active surveillance system by using telephone interviews to collect information about the reasons for absences, categorized absences as illness related and nonillness related (including injuries), and classified illness-related absences into gastrointestinal and respiratory categories. To ensure adequate parental recall of events associated with the absence of interest, interviews were conducted only for those absences reported within 4 weeks of occurrence. Of the 3,294 absences reported within 4 weeks, 536 were classified as nonillness related based on school reports, and 2,758 absences required telephone follow-up.
Parents were contacted after each absence reported within 4 weeks to inquire whether it was illness related and, if so, what the symptoms were. Each illness-related absence was classified as respiratory or nonrespiratory on the basis of the reported symptoms. A respiratory illness was defined as an illness that included one or more of the following symptoms: runny nose/sneezing, sore throat, cough (any, wet, or dry), earache, wheezing, or asthma attack. Respiratory absences were further classified into non-mutually-exclusive categories of upper respiratory and/or as one of two types of lower respiratory illnesses: lower respiratory illness with wet cough or lower respiratory illness with wet cough/wheeze/asthma. An upper respiratory illness was defined as a respiratory illness that included one or more of the following symptoms: runny nose/sneezing, sore throat, or earache. Gastrointestinal-related illnesses included illnesses that included "stomach problems" such as vomiting and diarrhea as one of the reported symptoms.
Absence incidence rates
Each absence day was categorized as an incident or prevalent absence day by using absence reports and school calendars to identify the days on which each school was in session. An incident absence day was defined as one that followed attendance on the preceding school day. A prevalent absence day was defined as one that occurred after an absence on the preceding school day. Absences on Mondays were considered incident if the child attended school on the preceding Friday. The date of an absence occurrence was assigned to the incident day of each series of absence days.
The daily number of incident absences in each community and the corresponding daily number of children at risk of an absence in each community were used to calculate daily community-specific incident absence rates. The number of at-risk students attending a school was defined as the number of participants enrolled in a school on a day that the school was in session minus the number of prevalent absences. Daily community-specific incidence rates of absences were calculated by pooling the data from the reporting schools in each community and dividing the community-specific number of incident absences by the number of students attending schools in that community on the day of interest. The average incidence rate for school absences was computed for each community by averaging daily rates and, for the entire cohort, by averaging across days and communities. Stratified rates (e.g., by asthma status) were calculated by identifying the number of absences and number of students at risk within each stratum and calculating daily community-specific rates and average rates as described.
On the basis of data collected by the active surveillance system, absences were divided into three mutually exclusive outcomes: non-illness-related absences, illness-related absences, and absences of unknown type (due to failure to obtain necessary classification information). Because some absences were of unknown type, the type-specific absence incidence rates were adjusted for ascertainment failure. To adjust type-specific incident absence rates, a daily community-specific information success ratio was calculated, which was defined as the daily proportion of timely absence reports in each community for which sufficient information was obtained to assign the absence as illness or nonillness related. This success ratio was smoothed over time to reduce the random fluctuation due to the limited number of events on each day within a community, but in such a way not to substantially alter the overall trend in the data or the observed values. A symptom-specific incidence rate corrected for ascertainment was calculated as follows: (number of incident cases)/(number at risk x smoothed success ratio).
Statistical analysis
Poisson regression models that accounted for overdispersion were used to estimate the relative risk of school absences from ETS exposure adjusted for potential confounding covariates (14). The outcome variablethe observed number of absence countswas modeled as a function of ETS exposure and other individual-level covariates. The log-transformed value of the expected number of absence counts over the study period was included in the model as an offset term, essentially resulting in a log-linear model for the ratio of the observed and expected absence counts. The expected number of absences per subject was calculated by summing the community- and day-specific expected number of absences for the days that a given subject was at risk. The community- and day-specific expected number of absences per subject was obtained by multiplying the day-specific average rate of absences for all communities by the number of children at risk in the given community on the given day. The expected number of absences was corrected for incomplete ascertainment of absence type by using the community- and day-specific success ratio.
Poisson models were fitted to estimate the relative risk of absences for asthma and wheezing outcomes, and various ETS exposure metrics were used to adjust for potential confounders including age, sex, ethnicity, parental education, health insurance status, family income, body mass index, and time activity patterns. Children who reported smoking at least one cigarette in the previous month were excluded. Dose-response relations were assessed by using significance of linear terms for number of household smokers and model fit comparing dichotomous ETS exposure with models that included categories for number of smokers. Modification of the effects of ETS by asthma or wheeze status was assessed by using nested models and likelihood ratio tests. All analyses were conducted with the GENMOD procedure in SAS software (15).
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RESULTS |
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DISCUSSION |
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Our results provide additional support for and extend the conclusion of numerous studies that children with asthma are more susceptible than children without asthma to the effects of ETS (2, 3). The 4.5-fold increased risk of respiratory-illness-related absences among asthmatic children who were exposed to two or more household smokers shows that the effects of ETS may extend beyond increased symptoms or medication use to adversely impact a childs education and family functioning. In this study, the data were insufficient to assess the effects of medication use on absences among children with asthma.
Children without asthma who reported exposure to two or more household smokers also had an increased risk of respiratory-illness-related absence. Although the magnitude of the ETS risk was greater for the 15 percent of children with asthma, all school-age children may be at increased risk of respiratory-illness-related absences when exposed to ETS. Our results suggest that the social and economic burden from the morbidity associated with ETS exposure among children may have been underestimated.
We did not directly study the mechanisms by which ETS exposure affects absenteeism. ETS is a complex mixture of respiratory toxins that adversely affect immune function, airway function, and the respiratory epithelium broadly through several pathophysiologic pathways. It is likely that ETS exposure increases the risk of an absence by increasing the risk and severity of respiratory infections, severity of asthma airflow obstruction, and inflammation and symptoms. The higher risk from ETS exposure among children with asthma is consistent with increased respiratory infections that are the primary pathway for asthma exacerbations. Further research is needed to define the mechanism for ETS effects because such knowledge will be essential to developing interventions to protect children whose parents continue to expose them to ETS.
Although school absenteeism data have been used for a limited number of health studies, absences have been considered too nonspecific to be a broadly useful source of information for studies of childrens health. Consideration of the epidemiology of absenteeism suggests that such data offer opportunities for both research and public health surveillance. A number of population-based studies have documented the descriptive epidemiology of school absences. Reports based on the National Health Interview Survey and other surveys show that absence rates vary by school, age, grade, and gender and are likely to be affected by family structure, function, and other social factors (21, 22). Because a number of non-health-related factors influence absenteeism, it has not been widely used as a measure of the adverse effects of ETS or other exposures; however, the majority of school absences are illness related and are attributable to illnesses and respiratory infections (5, 21). Our findings support the use of carefully collected school absenteeism data for a broad range of public health purposes.
Our study enrolled more than 2,000 fourth-grade schoolchildren and their families. The active surveillance system and modeling strategy did, however, have some limitations. Although the restriction of absences to those reported within 1 month of occurrence and the incomplete ascertainment of type of absence may have introduced bias into our study, this method was adopted to minimize any recall bias of absence events by parents. On the basis of distributions of the study population in the full and restricted sample of absence days, we found little evidence of any selection bias from the restriction. To account for the effects of incomplete ascertainment, the denominator of the rates and the offset in the Poisson models were adjusted for the proportion of absences that included information on absence type. Our study also had limited information on ETS exposure assessment and asthma phenotype. Exposure to tobacco smoke was assessed retrospectively by using questionnaire responses and was not validated by objective measurements. However, exposure estimates based on questionnaire responses have been validated (2325). Asthma status was assigned by using parental reports of a physician diagnosis of asthma. Parental reports have been shown to reflect physician diagnoses; however, the diagnosis of asthma by a physician depends on access and use of medical care and on physician diagnostic practices (26, 27). However, the ETS effects were also observed in children with wheezing, indicating that a diagnostic bias was unlikely to explain our results.
In conclusion, household ETS exposure was associated with increases in respiratory-illness-related school absences in children aged 812 years. Because exposure to ETS is common, the substantial increased risk of school absenteeism from respiratory illnesses documents an important adverse impact of ETS on childrens health and well-being. The social and economic burden resulting from childrens exposure to ETS may be broader and larger than previously appreciated.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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