The potential for exposure to lead is more complicated than you may believe. According to a new study in Science of the Total Environment published last March by researchers at Macquarie University in Sydney, Australia, the size of dust in your home might affect how dangerous it is.

If you are like me, you think that beyond lead mines and old paint chips, there isn’t much risk of exposure to lead. Similarly, you might think that the issue of lead poisoning isn’t relevant anymore; the developmental delays, neurologic changes, and even fatalities caused by lead poisoning are no longer a concern. Professor Mark Patrick Taylor, co-author and part of a global network working to get to the bottom of what makes up the average person’s household dust, argues that this isn’t always the case.

“A lot of dust is skin,” Taylor says, in addition to “hair, carpet fluff, clothing fibers, pet hair, soil from outside [which] is rich in organic matter, leaves, etc.” However, Taylor is most concerned about the inorganic contaminants (or non-living parts of dust) in people’s homes. When you walk into your house, your shoes track in all the contaminants you may have stepped on outside, which becomes indoor dust. This is most concerning when the dust is contaminated with toxic elements, like zinc, arsenic, and lead. The chance for these toxic elements to get into your home depends on where you live. Any area with an industrial history, like many big cities and mining towns, has the potential for these metals to get into your dust.

But how much of that lead can actually get into your body? The answer may be in your vacuum cleaner. Sampling dust from Sydney residents’ homes, vacuum cleaner bags held the secrets the scientists were after. Their goal was to see how the size of household-dust affected the amount of lead your body might be able to take up, also called its bioaccessibility.

To do this, researcher Israel Doyi and his team collected over 300 vacuum cleaner bags from across Sydney, and sorted the dust into four different sizes:

• smaller than 45μm (the minimum width of human hair)
• 45-90μm (the width of the average human hair)
• 90-150μm (the thickness of paper)
• 150-250μm (the smallest pencil lead available, 0.2mm)

They thought that size might affect the amount of lead that enters your bloodstream, expecting smaller particles to be more dangerous. Even though you can’t see these dust particles, they are especially dangerous if you inhale or ingest them, going straight to the lungs and stomach and directly into your bloodstream.

In order to see how much lead the body might take up, researchers used simulated gastric fluid. This fluid recreates the complex chemical reactions that happen in our stomachs, and allows scientists to see how lead in dust interacts with our bodies. What they found was surprising.

The size of dust matters a lot. They were surprised to find that just because a piece of lead dust is small, that doesn’t make it more dangerous. Most significantly, they found that the very smallest (smaller than 45μm) and the largest dust (150-250μm) poses the greatest threat to our health. This knowledge is critical, because larger dust particles are much more common than smaller dust particles. Taylor summarized the finding, saying “if [people are] going to have any environmental exposures from environmental contaminants, it’s likely to be from dust.” When people spend up to 90% of their time indoors, these results are concerning, and require action.

This study should influence how we address lead exposure. Most health risk assessments assume that the risk of lead in dust is uniform, regardless of the size of the dust. This study shows otherwise. Regulators like the Environmental Protection Agency (EPA) or the Australian equivalent, the National Environmental Protection Council (NEPC), must take this study and the size of lead into account when calculating risk. Changes to risk assessment models would accurately redesignate sites as toxic and require new clean-up efforts.

Any final tips on how to keep your home clean? Taylor recommends using a wet mop or HEPA vacuum to clean, keeping separate indoor and outdoor rugs and regularly cleaning both. And next time you come back from a walk, leave your shoes at the door.