Harvard Dust Mite Study

By Harvard Health Publications

Anyone who has yearned for a clean house can attest to the seemingly supernatural properties of dust. No sooner have you banished it from the bookshelf than it peeks out from under the bed, shape-shifting from powdery and puffy to sticky and stringy. But the menace of dust can extend beyond the realm of housekeeping and damage health in serious ways. What's it made of?

Household dust is an amalgam of biological and nonbiological substances that either migrate from the outside environment or are generated by the human — and animal — residents of the home. About 60 percent of household dust comes from soil tracked inside on the bottoms of shoes or the paws of pets or as airborne particles that have wafted through doors, windows and other openings. The remaining 40 percent is a hodgepodge of substances that originate inside the dwelling, including skin cells, pet fur, insect residue, carpet fibers and kitchen grease.

Every home has its own dust profile that reflects the particulars of the surrounding geography, structure of the house, and activities of the inhabitants. Homes with pets tend to harbor dust that contains more hair and dander — small pieces of hair, skin or feathers that flake off animals. In dry, windy climates, people contend with layers of fine silt, and in cities, household dust often has a sooty quality from car exhaust and other sources of air pollution.

The unseen world

Dust can also be home to a thriving community of bacteria, fungi and minute spider-like insects called dust mites that feed on dead skin cells. We should be grateful that dust mites don't bite or spread disease directly. But the debris they leave behind can provoke a powerful allergic reaction. As many as 500 mites can live in a single gram of dust, each of them producing many allergy-inducing droppings.

Endotoxins, substances released from the cell membranes of dust-dwelling bacteria, are another biological component of dust. A large study conducted by the National Institute of Environmental Health Sciences evaluated endotoxin levels in over 2,500 dust samples from homes around the country. Results showed that adults who live in residences with increased concentrations of endotoxins reported higher rates of wheezing and other asthma symptoms. The effect was most pronounced in cases where endotoxin levels were elevated in bedrooms.

The hygiene hypothesis

Some research supports the idea that exposure to certain elements in dust early in life might have some benefits. According to the hygiene hypothesis, as it is called, more people have allergies these days because they grew up in relatively clean urban and suburban environments, not on farms full of endotoxins and dander from animals. As a result, their immune systems aren't challenged and don't develop properly, so parts of the immune response become hypersensitive to certain allergens (any protein can be an allergen). Epidemiological studies have found interesting correlations between exposure to livestock early in life and having fewer allergies later on. As for pets, a review that included 27 studies concluded that growing up with one was associated with a slightly lower chance (14 percent) of developing allergies. When exposure was separated by dogs versus cats, exposure to dogs was associated with more protection.

The danger

The indifferent housekeeper may protest that a little dust never hurt anyone. But a lot of dust — and even a little, depending on the contents — might. On average, people spend more than two-thirds of their time at home inside, so we spend a lot of time in and near the dust in our homes. The most vulnerable family members are the youngest: Infants, whose organs are still developing, are up to 100 times more susceptible to the health hazards of dust-borne pollutants than adults are. Dust and the toxins it contains enter the body in one of two ways — people breathe it in or swallow it. Very small particles floating in the air (2.5 microns or less in diameter) can be inhaled deeply into the lungs and get into the blood. They've been associated with heart attacks and a host of other cardiovascular problems. Larger particles — the stuff we think of as dust — can cause serious, even life-threatening, problems for people with asthma and allergies. For most people, though, many dust particles are too large to get deep into the lungs. So in most cases, the bigger danger is probably from swallowing dust. But who does that? Young children, of course, put many things into their mouths. Even in clean homes, talcum-size dust particles float around and land on surfaces and objects of oral interest to the young. Dust particles that size can also adhere to adult hands and be transferred to food inadvertently as we eat and cook.

Dusty dwellings

Not surprisingly, the amount of soil that enters a house goes up with the number of people living there. The traffic patterns of the inhabitants are important as well. One study showed that indoor dust levels doubled when the children spent more hours playing outside in the summer months. Houses with pets amass significantly more dust, too, both from the dirt the animals track in and the hair and dander they slough off. Dog breeds that shed very little, like poodles and Portuguese water dogs, will mean less dust. However, because allergies are caused by dander from the animal's skin, not the fur itself, there's still a risk of a respiratory reaction. There is no such thing as a completely nonallergenic dog or cat. Ash particles from a wood-burning stove or tobacco smoke also increase the dust quotient, as well as fibers from carpeting and plush upholstery.

What you can do One simple way to stem the amount of soil coming into a house is to put heavy-duty doormats in front of the doorways. Even better: Remove your shoes before coming inside, and ask visitors to do the same. Installing weatherproofing around doors and windows will help keep airborne particles out — and lower your electric bill. Filters on heating and air-conditioning systems will probably also cut down on dust, although larger particles will settle out of the air and onto surfaces before they get a chance to be filtered out. Some companies identify the minimum efficiency reporting value, or MERV, score of the filters they sell. The higher the MERV, the smaller the particles captured by the filter. High MERV filters get expensive, costing $50 or more, and filters should be changed every three months or so that a heating or air-conditioning system is in use.

Portable air cleaners with high-efficiency particulate air (HEPA) filters are another option. HEPA filters are made with a very fine gauge of material that filters out extremely small particles.

Most of the machines sold as air purifiers work by applying an electrostatic charge to dust particles that draws them to metal plates within the unit. In addition to perhaps being less effective than HEPA filtration, air purifiers may emit small amounts of ozone, a gas that can worsen asthma symptoms and may have other bad health effects. Many of these machines now have ozone-to-oxygen converters designed to decrease the amount of ozone emitted.

But perhaps the single most effective way of controlling dust levels involves nothing more complicated than routine housekeeping or scheduling a professional dust mite eradication service at least 2-3 times per year. While regular vacuuming, preferably with a vacuum cleaner with a HEPA filter, is a good place to start. University of Arizona researchers found that vacuuming with a standard household vacuum removed only a small percentage of the arsenic-containing floor dust in the homes they tested, which is why a more powerful and comprehensive service may be needed. Although carpeted surfaces can look clean after a few passes with a household vacuum cleaner, dust often lurks deep within the pile. Regular household cleaning does relaunch settled dust, and the longer the interval between cleanings, the thicker the dust layer, and the greater the risk. Copyright © 2010 by the Presidents and Fellows of Harvard College. Used with permission of StayWell.

All rights reserved. Harvard Medical School does not approve or endorse any products on the page. Harvard is the sole creator of its editorial content, and advertisers are not allowed to influence the language or images Harvard uses.