by Allan Chen
Those factors in the home that cause or exacerbate asthma and allergies in children are not well understood, but scientists agree that they include allergens like dust mites, mold, and environmental tobacco smoke.
Mark Mendell, an epidemiologist in Lawrence Berkeley National Laboratory's (LBNL) Environmental Energy Technologies Division, suggests another set of factors worth investigating: emissions from common indoor building and decorating products, such as composite wood materials that emit formaldehyde, flexible plastics that emit "plasticizers," or even a fresh coat of paint. In a paper for the journal Indoor Air, Mendell conducted a review of epidemiologic studies published in scientific journals from 1989 through mid–2006, comprised of 21 studies mostly from outside the United States. These studies found associations between common indoor materials and an increased risk of asthma, pulmonary infections, and allergies in children.
The growing body of research identified specific risk factors, including organic chemicals like formaldehyde, benzene, and phthalate esters (the "plasticizers" that make some plastics flexible); indoor materials, including carpet, paint, flexible flooring, and other plastics; and various activities related to installing and cleaning these materials indoors.
Mendell is careful to note that "causal relationships have not been demonstrated" in these studies. However, his review of these studies, most of which were conducted in Europe, suggests that U.S. scientists should take a closer look at emissions from indoor materials for their possible effects on children's health.
Asthma Prevalence Growing
According to the Centers for Disease Control (CDC), the prevalence of asthma in children in the United States increased from 3.6 percent in 1980 to 6.2 percent in 1996. This is an average increase of 4.3 percent per year, a relatively high growth rate that worries health experts. Asthma can be a severe, life-threatening illness; moreover, both allergies and asthma are expensive to individuals and to society. Current science suggests that risk factors for developing asthma include genetic predisposition; specific allergens such as dust mites, cockroaches, and pet dander; moisture and mold; and environmental tobacco smoke. There could be other risk factors, as yet unknown.
A complication for researchers seeking the causes of the asthma growth rate is that while a few risks have been clearly demonstrated, the case for others is weak. "Sufficient evidence of a causal relationship" is the CDC's strongest level of evidence; "sufficient evidence of an association" and "limited evidence of an association" are weaker. The CDC also draws a distinction between factors that cause a condition, such as the development of asthma, and those that exacerbate an existing condition, for example by triggering asthma attacks.
In an authoritative report published in 2000, the Institute of Medicine, part of the National Academy of Sciences, concluded that there is sufficient evidence for a causal relationship between the development of asthma in susceptible children and exposure to a house dust-mite allergen. The report also concluded that there is sufficient evidence of an association between exposure to environmental tobacco smoke and development of asthma in younger children, a statement strong enough to suggest that parents should protect children from exposure to tobacco smoke. Except for environmental tobacco smoke, however, the CDC currently does not recognize the association of chemical agents encountered in the home with asthma.
"The most frequently identified risk factors related to indoor residential chemical emissions include formaldehyde or formaldehyde-emitting particleboards, plasticizers or plastic materials, and recent painting," said Mendell.
Pressed wood products include particleboard, medium density fiberboard, interior plywood, and interior hardwood paneling. The urea-formaldehyde resin within them releases formaldehyde over time. So do other indoor sources, including tobacco smoke, varnishes, paints, and carpets. Moisture on building materials can also accelerate the release of airborne formaldehyde. Painting and freshly painted surfaces release various volatile organic compounds into indoor air.
Formaldehyde emissions and particleboard were associated with asthma, chronic bronchitis, and other respiratory symptoms. Phthalate plasticizers, or the presence of phthalate-containing surface materials such as polyvinyl chloride or vinyl, as well as painting and other room renovation, were also associated in various studies with asthma, allergy, and respiratory symptoms.
The studies that examined indoor material emissions differed widely in design and focus, said Mendell. Their methods varied from measuring actual chemical concentrations of indoor air or dust to simply observing the presence or absence of materials that emit organic chemicals. Thus, there is a need for more rigorously controlled research to eliminate possible confounding factors; for example, other unmeasured factors could be the real explanation for the findings in some studies.
"Future studies will have to carefully measure formaldehyde and other chemicals indoors," said Mendell. "They will need to determine whether it's a specific chemical such as formaldehyde that causes the association, or some other emission that is always associated with the presence of that chemical. Formaldehyde emissions, for example, are known to come from particleboard, but this material also emits other chemicals."
One chemical compound, or several working together, could be the cause of health conditions. Still, when all the studies are taken together, and considering both their strengths and weaknesses, "it is hard to imagine what else could explain these findings that is not related to indoor chemical emissions," Mendell said.
Mendell has been studying the associations between health and indoor environmental factors throughout his career. As an epidemiologist with the CDC's National Institute for Occupational Safety and Health, he studied the indoor factors associated with "sick building syndrome," a set of respiratory and other symptoms among workers in office buildings. Detailed by the CDC to LBNL in 2000, he continued to work on those issues and ultimately joined the staff of LBNL's Indoor Environment Department.
"I believe that studying health impacts of the indoor environment is one of the most important directions for building science," he said. Most employed people work indoors and indeed spend most of their lives indoors, yet the health effects of the indoor environment are not well understood. LBNL's Indoor Environment Department is one of the few research groups in the U.S. currently doing research in this field.
It was while considering literature relating indoor environments and respiratory health effects in children that Mendell noticed the large number of papers from outside the United States that seemed to implicate emissions from building materials. After carefully reviewing the available evidence, he said, "These studies justify conducting further research in this area, especially where the evidence is strongest — as with formaldehyde at levels commonly found in homes."
