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June, 2007
Behind Closed Doors: Beware the Air

Those factors in the home that cause or exacerbate asthma and allergies in children are not well understood, but scientists agree they include allergens like dust mites, mold, and environmental tobacco smoke.

Mark Mendell, an epidemiologist in Berkeley Lab's 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.

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In addition to known allergens like mold, tobacco smoke, and dust mites, children's allergies may also be exacerbated by emissions from composite materials, the chemicals that make plastics flexible, and even paints.

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 found associations between common indoor materials and 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, 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 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 association of chemical agents encountered in the home with asthma.

Building materials are emitters

"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," says Mendell.

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Some common household substances and materials emit chemicals like those diagrammed here, including formaldehyde, benzene, phthalate esters, polyvinyl chloride, polyurethanes, and epoxy resins.

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, says 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," says 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 particle board, 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 says.

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 Berkeley Lab in 2000, he continued to work on those issues and ultimately joined the staff of the Lab'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 says. 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. Berkeley Lab'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 says, "These studies justify conducting further research in this area, especially where the evidence is strongest — as with formaldehyde at levels commonly found in homes."

What should concerned parents do?

Currently there is not, in the CDC's phrase, "sufficient evidence of a causal relationship" between respiratory problems in children and building-material emissions. Yet some parents may want to take steps to minimize risks to their children before a scientific consensus on this question has formed. What are reasonable steps to take?

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Strategies to reduce possible risks from chemical emissions include using natural bedding like feathers, which also harbor fewer dust mites, plus furniture made of real wood and flooring of real wood or authentic linseed-oil-based linoleum.

"I would think twice before repainting an infant's nursery or using pressed wood products in children's rooms," says Mendell. "Also, in the case of an asthmatic child, I would avoid wrapping the child's mattress in a vinyl product."

Wrapping a child's mattress and pillows is often recommended as a control measure to minimize factors that exacerbate asthma, because dust mites and other organisms known to be associated with biological risk factors tend to take up residence there. However, some of these wrappings are made of vinyl that emits phthalates over time, and the emissions could pose respiratory health risks to children.

Despite the common practice of providing children at risk for asthma with synthetic pillows and comforters, synthetic bedding has been consistently associated with greater occurrence of respiratory health effects than natural bedding like feathers. It is not yet clear if this is due to chemical emissions from synthetic bedding or to the enormously higher growth rate of dust mites in synthetic bedding than in feather bedding — a surprising finding, yet one demonstrated by substantial research.

Parents could also avoid putting particleboard furniture into children's rooms. Particleboard is sometimes covered in wood-grained vinyl in an attempt to reduce formaldehyde emissions, and the extensive use of these materials in mass-produced furniture means extra work for parents looking for furniture free of them. Alternatives are available, however.

Mendell also suggests choosing alternatives to PVC flooring for children's rooms — for example, real wood or authentic linoleum. Product selection can be tricky. For instance, most bamboo flooring, widely promoted as a "green" flooring material, contains the same formaldehyde-releasing glue as particleboard. Some manufacturers of bamboo, however, use the non-formaldehyde-emitting glues required for all composite wood products sold in Europe and Japan. The U.S. has not placed the same emphasis on controlling indoor emissions for health.

Mendell's hope is that further research in the U.S. will quantify these health risks to children, and to adults as well, helping guide informed consumers to take health-protecting actions.

Additional information