While reading a new study surveying insect diversity inside American homes a few months ago, I was struck by this map:
The Great American Desert could be the Great American Dust Mite Desert, apparently. Actually, it’s not that simple, as I wrote in a short piece for the Advances section of this month’s issue of Scientific American.
Dust mites are one of the most common causes of asthma, but they may also provoke other allergic responses. They feed on bacteria, fungi and, unfortunately for us, the teaspoon of dead skin scales we slough every month. The warm, humid environment of a bed (don’t forget you sweat in there) is a dust mite happy hunting ground.
What the map actually shows is where dust mites are super abundant. Dust mites do live in the Great Plains and mountain west, but there are far fewer here than elsewhere due to the lack of humidity (dust mites cannot drink and must absorb moisture from the air). Very few survive if indoor temperatures are lower than 71°F and relative humidity is less than 45%, according to a 2013 paper. As a result, high and dry regions are relatively dust-mite free, while humid climates on the coasts and in the eastern U.S. produce bumper crops of mites.
But as I uncovered while reporting the piece, the back story is even more interesting. According to David Miller, a scientist at Carleton University in Ottawa, who studies the links between damp housing and health, until the late '60s and early '70s, dust mites were rare in North American homes. Prior to that time, houses were leaky and cold in winter, un-air conditioned in summer, and in western interior North America(WINA -- see? still useful!), also exceedingly dry. When Miller entered the field, he said, dust mites were present in most houses, he said, but not in high numbers.
What changed was our homes. In order to increase comfort and save energy, we tightened them up. In winter, we heated them more. In summer, we installed air conditioning and swamp coolers, which can create cold surfaces that condense moisture from the air. In short, we made our homes warmer and more humid – and ideal habitats for the dust mite. Carpet, mattresses, and pillows are particularly fine mite homes because they can trap both humidity and dust.
Yet it was only in the 1960s that Dutch and Japanese scientists proposed that house dust mites were an allergen, Miller says – and people laughed. It wasn’t until 1988 that there was even a consensus that they cause allergies. Now, dust mites are nearly ubiquitous, and not uncommon even in WINA. That’s because people often introduce clothing and furniture carrying dust mite stowaways from more humid parts of the country, but also because air conditioning and swamp coolers produce cold surfaces that condense moisture, potentially allowing dust mites to thrive. Three decades ago dust mite allergies were very uncommon to unheard of in WINA. That is no longer the case. Some 84% of U.S. dwellings are now estimated to have detectable levels of dust mite allergens.
The scientists who produced the map at the top of this post didn’t actually set out to study dust mites. They just wanted to document the diversity of insects inside homes across America in a way that didn’t involve entomologists spending hours with tweezers and a backache inside each home. Instead, they had people dust the tops of their door frames with cotton swabs, and then pack the samples off to the lab. In this way, they were able to easily and cheaply get a vast library of data on the insect occupants of American homes for the first time.
In the lab, the scientists amplified the DNA in the samples and then sequenced a short segment of a gene called cytochrome c oxidase subunit I in the 732 dust samples collected by citizen scientists. Differences in the gene reveal taxonomic differences between insects.
In general, their findings were revealing but not surprising. They found homes with pets tend to host a greater diversity of bugs – not necessarily more, though, just more kinds. Having a basement increased insect diversity – not a news flash for occupants of homes with basements or – especially -- anyone who’s ever had to live in one. Homes in rural areas also boasted greater insect diversity, likely due to their closer proximity to nature.
However, they did find a host of insect occupants people may not expect. Although cockroaches, flies, ants and termites are pests people commonly associate with homes, said Anne Madden, a postdoctoral researcher and microbiologist at North Carolina State University, they also uncovered an entire food web based on planthoppers and aphids (Madden called them “the antelope of the savanna of our homes”) living on house plants, complete with aphid-killing parasitic wasps (the lions).
It was to their great surprise that they discovered they could detect dust mites in door frame dust. They had not expected to. Dust mite allergens were not supposed to be on hard surfaces, especially those like door frames far from carpet and mattresses, because the allergens occur in heavy particles that don’t stay airborne long, Madden told me.
On the other hand, every time we walk across a carpet, we stir up an invisible cloud of dust just like the Peanuts character Pig-Pen, Miller said, which is one reason why doctors so aggressively recommend using vacuums fitted with HEPA filters (other reasons: dust is hygroscopic, attracting moisture that helps mite, and dust is of course full of mite chow as well).
In any case, in places that teem with dust mites, the simple act of walking around seems to be sufficient to get detectable levels of dust mite DNA all the way up to the top of door frames, where no respectable dust mite would dare to tread. In places where mites are scarcer? Not so much.
The takeaway: wherever you live, if you suspect house dust mites cause you problems, stick it to them by taking away their favorite thing in the world -- dust.
Madden, Anne A., Albert Barberán, Matthew A. Bertone, Holly L. Menninger, Robert R. Dunn, and Noah Fierer. "The diversity of arthropods in homes across the United States as determined by environmental DNA analyses." Molecular Ecology (2016).