Forget lead paint in toys. Canadians may be exposed to a much wider variety of metals, chemicals and pesticides on a daily basis as the result of a seemingly harmless domestic nuisance: house dust.
It's a potential health hazard that scientists are only beginning to understand. Health Canada and the Public Health Agency of Canada have embarked on a landmark four-year national study to determine levels of metals and chemicals in house dust across the country, and how health risks should be addressed.
Dust typically contains a variety of toxins released by common household products including plastics, electronics, furniture, garden soil and lead paint, which is common in older homes.
Scientists don't know for sure where all the toxins commonly found in dust come from. They also don't know the typical concentration of metals and other toxins in house dust.
Unlike lead paint on toys or chemicals in plastic, scientists say house dust presents a unique risk because the loose, free particles can be easily ingested by children crawling on the floor or may be inhaled when dust becomes airborne.
Research has linked lead, flame retardants and pesticides found in house dust with the accumulation of these toxins in children's bodies. "We've found high levels of lead in house dust is associated with higher blood lead levels in kids," said Paul Lioy, director of the exposure science division of the Environmental and Occupational Health Sciences Institute, jointly sponsored by Rutgers University and the University of Medicine and Dentistry of New Jersey.
One of the major questions federal government researchers are focusing on in their study is why indoor house dust often has significantly higher levels of lead and other metals than outdoor soil or water.
It's a baffling phenomenon that may have significant implications, because it could mean that soil cleanups and other measures don't address some of the major sources of human exposure to metals.
"There's a lot we don't know about house dust and what makes it potentially worrisome is just the number of chemicals and various things that end up in house dust," said Kapil Khatter, an Ottawa-based family physician and pollution policy adviser for Environmental Defence, a national advocacy group.
Scientists do know that dust is the main source of human exposure to polybrominated diphenyl ethers, used widely as flame-retardant additives in electronics and furniture. These chemicals can accumulate and remain in a person's system for years, according to a study published in 2005 by the American Chemical Society.
"What we suspect is that because some of these chemicals are neurotoxic, they can then influence your brain and how your brain works," said Miriam Diamond, an environmental science professor at the University of Toronto and lead author of the study.
"Toddlers and little kids pick up more dust than we do and they're more vulnerable to these effects."
Children's brains are still developing, and toxic substances can interfere with normal growth, she said.
For adults, "dust is a significant part of total exposure for certain kinds of contaminants," said Ruthann Rudel, senior scientist and toxicologist at the Silent Spring Institute, a non-profit research organization based in Massachusetts. Ms. Rudel conducted a study that found pesticides and flame retardants in Cape Cod homes.
In their house dust study, researchers from Health Canada and the Public Health Agency of Canada will test house dust samples from 1,040 randomly selected detached homes across 13 Canadian cities, including Barrie, Ont., Hamilton and Thunder Bay.
There are currently no limits or regulations for levels of toxic substances in house dust, and Health Canada cautions that the presence of metals and chemicals in dust doesn't necessarily mean individuals will be exposed to harmful levels of the substances. But researchers conducting the federal study will alert homeowners when they detect high levels of lead or other harmful materials.
Health Canada plans to make the results public once the study is complete.
Pat Rasmussen, research scientist at Health Canada and principal investigator of the national house dust survey, led a study in 2000 that found dust in 50 Ottawa homes contained "significantly higher concentrations" of lead, mercury and cadmium than outdoor soil.
Although levels varied widely, researchers detected lead in house dust at several hundred to several thousand parts per million. By comparison, federal limits for lead in most toys is 600 parts per million.
One of the best ways to eliminate the potential hazards is to keep floors and other surfaces clean and dust-free, said Dr. Rasmussen. It's also important to remove footwear before walking around the house, since outside soil may contain metals that can be tracked through the house.
Keeping a clean house won't stop the release of lead or flame retardants into house dust. But until researchers have a better understanding of how dust becomes contaminated and the level of risk it poses, it's the best solution to a perplexing problem.
"I think it won't be long until more studies have the ability to relate some of these toxic chemicals with a wide range of behaviour effects," Dr. Diamond said.
How does dust become toxic?
Scientists are still trying to understand what in our environment contributes to toxins in house dust and how dust becomes contaminated. But researchers say they believe common household products release toxins into household dust. Here are some potential common sources:
carpets and consumer
Many of these products contain polybrominated diphenyl ethers, commonly referred to as flame retardants, which are a group of chemicals that may cling to house dust when released. Some evidence suggests the chemicals can cause reproductive and neurological damage. Environmental advocates have long argued for a ban on flame retardants and say their presence in house dust poses a significant health threat.
Many older homes may still contain lead paint, which could be released into house dust and possibly be ingested or inhaled by humans. Studies have show that even low levels of exposure to lead can have an impact on a child's brain.
Researchers have found the presence of pesticides in house dust, which could occur when shoes are worn in the house. Research suggests exposure to pesticides could contribute to diseases and affect a child's neurological functions.
Garden soil and street dust
Several studies have shown house dust contains high levels of lead and other metals, leading scientists to believe outside soil tracked into the home could be a major source of harmful metals including mercury and cadmium.