The Dust That We Breathe Inside Our Homes

George Carlin: Dusting is a good example of the futility of trying to put things right. As soon as you dust, the fact of your next dusting has already been established.

Have you ever created a dust cloud by plopping down on a couch, or gazed at dust particles floating in a shaft of sunlight from the window? Dust particles are constantly settling onto surfaces in our homes, only to become resuspended again every time we sit down or walk around, and whenever we vacuum carpets, sweep the floors, or make the bed. These are just a few of many everyday activities that cause the redistribution of dust particles into the air that we breathe. 

The chemical composition of dust particles in typical urban homes has been the focus of a nationally representative study conducted by Health Canada researchers, entitled “The Canadian House Dust Study.” The study’s first priority was to quantify lead contamination in dust, which can be a significant source of exposure for toddlers and pre-schoolers who accidentally ingest dust through normal hand-to-mouth activities. Later the Canadian House Dust Study examined a host of other inorganic and organic chemical compounds that accumulate in dust after being released from consumer products, furniture, building materials, coatings, combustion products, tracked-in soil and infiltrated ambient particles. Results for hundreds of compounds have been reported in a series of publications which collectively show that the ability of settled house dust to accumulate chemical compounds makes it an excellent indicator of residential contamination.

While accidental ingestion of lead and other chemicals in settled house dust can be a significant exposure pathway, researchers are starting to recognize the importance of inhalation exposures caused by breathing resuspended house dust. Most of what we know about indoor dust resuspension processes comes from studies of human exposures to allergens such as dust mites and pet dander.

Recently, Health Canada researchers questioned how the chemical composition of indoor air particulate matter (PM) is related to the composition of settled house dust. They first conducted lab experiments using archived Canadian House Dust Study samples, which showed that resuspended dust particles in the <10 µm size range (known as PM₁₀ or the “inhalable fraction”) tend to have higher trace element concentrations than the bulk dust samples (<80 µm size range) from which the PM₁₀ was derived. Then they compared the elemental composition of samples of dust collected from living room floors in Windsor, Ontario with samples of airborne particles collected in the same rooms.

The results from the Windsor study confirmed that many trace elements are found in higher concentrations in airborne PM₁₀ than in settled floor dust from the same room. Most importantly, the study showed significant correlations between elemental concentrations in the matched PM₁₀ and settled house dust samples. Those correlations between elements in PM₁₀ and settled dust provided new evidence that we do indeed inhale resuspended house dust in our homes. The researchers concluded that house dust makes a very useful sampling medium for investigating inhalation exposures in indoor residential environments – not just exposures to metal compounds but also to the myriad of synthetic organic chemicals such as plasticizers, flame retardants, and pesticides that are found in house dust. 

Additionally, the study showed that concentrations of many elements are elevated in the inhalable fraction (<10 µm) of settled dust from heavily carpeted homes compared to homes with little or no carpeting. This observation is particularly relevant for understanding inhalation exposures because higher rates of resuspension are associated with carpets than with bare floors.

Another factor is the location of the home: previously the Canadian House Dust Study had demonstrated that indoor dust deposition rates are significantly higher in homes located near urban industrial zones than in homes in residential neighbourhoods located more than 2 km away from industry. In other words, that annoying film of dust on our bookshelves and coffee tables collects a lot faster depending on where we live. This observation about the influence of home location is also relevant to understanding inhalation exposures, because dust resuspension rates are higher in homes with greater amounts of settled dust.