Associations between indoor environmental factors and parental-reported autistic spectrum disorders in children 6–8 years of age
Introduction
Over the past few decades, our understanding of the genesis and character of autism has undergone a series of transformations. It is now viewed, not as a unitary disorder, but as one with multiple dimensions (hence the term, autism spectrum disorder, ASD), displaying many degrees of severity, and attributable to a variety of potential causes. We now recognize that its manifestations touch many organ systems as well as the brain. We still view it as rooted in genetics, but subject as well to environmental events and circumstances. Exposures to aberrant environmental conditions and to chemical contaminants, in particular, are now seen as possible factors in its etiology.
The current report emphasizes one class of conditions and contaminants; in essence, those arising from the indoor environment, which could be relevant from an exposure point of view since we spend most of our time in such environments. The report is one of a series directed originally at how certain properties of the indoor environment such as moisture-related problems and different building materials are associated with allergies and asthma in Swedish children. Almost adventitiously, the investigation uncovered an apparent association between certain of these properties and medically diagnosed ASD. The data are far from conclusive. They are puzzling, even baffling, and not readily explicable at this time. However, because they are among the few clues that have emerged about possible environmental contributions to autistic disorders, we believe that they should be weighed carefully and warrant further study.
The global incidence of allergy and asthma has risen substantially over the past three decades (Beasley, 2002, Beggs and Bambrick, 2005), as it has in Swedish children. One possible factor contributing to this rise, noted repeatedly in the literature, is the character of the indoor environment and exposures occurring in that setting. Because moisture-related problems or “dampness” have been associated with respiratory symptoms and with allergy and asthma in numerous studies (reviewed in Bornehag et al., 2005b) a consortium of Swedish organizations and institutions undertook an epidemiological study designated the “Dampness in Buildings and Health” study (DBH). DBH began in 2000. Its central aim is to determine elements of the indoor environment associated with asthma and allergic symptoms among small children and their parents. The choice of young children as the study population arose from two considerations. First, preschool children display a higher incidence of allergies than adults. Second, most of their time is spent in the home, the environment that is the focus of the effort. Children are also more vulnerable due to a relatively higher intake of food and a higher breathing frequency.
The data reported here represent one set of DBH findings. They describe associations between ASD in children aged 6–8 years and a number of environmental factors, including exposure conditions when they were 1–3 years of age and, almost surely, during pregnancy and the first year of life.
Section snippets
Method
The Dampness in Buildings and Health (DBH) study has so far embodied three phases.
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DBH-I, a cross-sectional study based on a questionnaire sent to parents of 14,077 children in the county of Värmland in 2000 (Bornehag et al., 2004a).
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DBH-II, a nested case–control study of 198 symptomatic children and 202 healthy controls from DBH-I (Bornehag et al., 2004b).
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DBH-III, a 5-year follow-up study of those eligible for DBH-I, conducted in 2005.
The data in the current paper are based on the initial
Results
Of the 4779 children in the study, 50% were girls (n = 2391) and age was evenly distributed among the participants (Table 1). A variety of changes occurred in family conditions and circumstances between 2000 and 2005. In 2000, 81% of the families lived in a single-family house; 5 years later this figure had increased to 86%. The proportion of homes larger than 150 square meters increased from about 21% to 28%. The most common type of flooring in the child's and the parent's bedroom in 2000 was
Discussion
At this time, we lack any firm knowledge about the extent to which environmental chemicals or other environmental conditions might influence the etiology or manifestations of ASDs. That is, using the terminology of cancer risk assessment, do they act primarily as initiators, or primary causes, or are they more like promotors, simply enhancing or amplifying a process already underway, or is it just a matter of confounding? For other neurodevelopmental endpoints, the past few decades have
Implications and conclusions
Our findings, and the questions they evoke, may be listed as follows:
Phthalate exposure early in development appears linked to ASD.
Is the link due to its association with allergies and asthma? Is it an independent factor, acting perhaps via endocrine disruption? Or is it a confounding phenomenon not yet identified? Biomarkers such as urinary metabolites would be a useful source of answers.
Other indoor environmental variables such as dampness, are also linked to ASD.
Is ventilation rate in the
Conflict of interest statement
None.
Acknowledgements
The study was funded by grants from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas), Swedish Asthma and Allergy Association's Research Foundation, and the Swedish Foundation for Health Care Sciences and Allergy Research. The participation of B. Weiss was supported in part by NIEHS grants ES013247 and ES015509 and Center grant ES01247.
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