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Reducing Biological Pollutants in Household Living Areas

Fungi (molds and mildew), bacteria, viruses, pollen from plants, animal dander, human skin cells and dust mites are some of the common biological pollutants inside the home. Some people experience allergic or asthmatic reactions to the contaminants in the air in their homes and need to be aware of how to reduce their exposure to these contaminants. The most severe reaction to allergens is an asthma attack which can be life threatening. The American Lung Association reports that there are nearly 10 million people in the U.S. with asthma. "Exposure to house dust mites, animal-related allergens (animal dander and cat saliva), and mold have been estimated to cause 200,000 or more emergency room visits a year by asthma patients (EPA)." Dust mites have been identified as the single most important trigger for asthma attacks.

While pollutant levels from a single source may not be a health risk, some homes have many sources that contribute to indoor air pollution. People who are inside a great deal may be at greater risk of developing health problems, or having problems made worse by indoor air pollutants. When a family member has asthma or allergies, keeping the home free of allergens presents some special challenges. The quality of indoor air can have a great effect on keeping allergies and asthma under control. Therefore, breathing clean indoor air can have an important impact on health.

There is no simple way to sample the air in your home to determine the level of biological pollutants. The amount of most biological substances required to cause disease is unknown and varies from one person to the next.

Fortunately, there are ways to control or eliminate most pollutants at a relatively low cost. Some cleaning techniques can actually contribute to the contamination of our indoor air while other techniques are important for maintaining good indoor air quality. Properly cleaning and maintaining your home can help reduce the problem associated with biological pollutants. There control measures will help you to achieve a healthy house.

Key points to understand are: 1) what is indoor biological pollution, 2) whether your home or lifestyle promotes its development, and 3) how to control its growth and buildup.

What are Biological Pollutants?

Biological pollutants are or were living organisms. They can travel through the air and are often invisible. Fungi (molds and mildew), infectious agents (bacteria and viruses), pollen from plants, animal dander, cockroach parts, human skin cells and dust mites are some of the common biological pollutants found inside the home. Even a spotless home may permit the growth of biological pollutants. Two major factors that create conditions for biological pollutants to grow are: 1) nutrients and 2) constant moisture with poor air circulation.

Conduct Inspection by Doing a "Walk Through" to Identify Source of Pollutants

Begin with a tour of your home. Follow your nose, and use your eyes. Look for conditions where biological pollutants can and likely will grow. The diagram below shows some of the places where biological contaminants may be found in the home.

  1. Dirty air conditioner
  2. Dirty humidifier and/or dehumidifier
  3. Bathroom without vent or window
  4. Kitchen without vent or window
  5. Dirty refrigerator drip pan
  6. Laundry room with unvented dryer
  7. Unventilated attic
  8. Carpet/rug on damp floor
  9. Bedding
  10. Closet on outside wall (cold wall)
  11. Dirty heating/air conditioning system
  12. Dogs or cats
  13. Water leaks and/or damage (around windows, the roof or the basement)

Source: Biological Pollutants in Your Home, Consumer Product Safety Commission and the American Lung Association, 1990.

Many of the places where biological pollutants are found have high humidity levels or moisture associated with them. Dust mites thrive in bedding, carpeting, and upholstery, especially when humidity levels are high.

What is the moisture level in your home?

Does your home feel humid? Can you see moisture on the windows or on other surfaces such as walls and ceilings? What is the usual temperature in your home? Have you recently had water damage? Is your basement wet or damp? Is there any obvious mold or mildew? Does any part of your home have a musty or moldy odor? Is the air stale? Do you have air conditioners or humidifiers that have not been properly cleaned? Do house plants show mold?

Are there nutrients available?

Do you have pets? Does your home have cockroaches or rodents? Is there dust or construction material(s) around? Is there firewood? Do you have carpets?

Mold and Mildew

Mold grows on organic materials such as paper, textiles, grease, dirt and soap scum. It requires moisture or high humidity. When a mold colony has been established, such as on a bathroom wall, it generates mold spores that float through the air, land on other surfaces, and, if the conditions are right, form new colonies.

Mold can also grow in standing water, such as in the reservoir of humidifier or dehumidifier. Mist from humidifier can spread the mold through the house.

Common places to find mold growing:

Mold can often, but not always, be seen growing on surfaces or textiles and can often be detected by its musty odor. It is the mold spores in the air that cause allergic reactions in sensitive people. Surfaces that are kept clean and dry are not likely to have mold growth.

Dust Mites

Dust mites are microscopic animals found in dust. They need a food source of dead human skin cells and high humidity levels. Dust mites often thrive in:

When these textiles are disturbed, such as during vacuuming, making beds, or walking across carpeting, the dust mite parts become airborne. Cleaning the surfaces where dust mite particles accumulate can reduce their concentrations in the air.

People who are sensitive to dust mites may need to replace carpeting in their homes with hard surfaced flooring and use area rugs that can be removed and cleaned. Removal of stuffed animals may be necessary.


Pollen is plant material that is usually generated by outdoor plants. It enters the house through open doors and windows, through cracks, on shoes and clothing, or can be brought in by pets.

Persons sensitive to pollen need to:


Persons allergic to pets are sensitive to the animal dander and saliva attached to dander and hair that is shed by their pets. The animal dander and hair accumulate most heavily in places where the animal sleeps such as on upholstered surfaces and carpeting. When these surfaces are disturbed, the particles become airborne.

Allergies to pets may be relieved by:

Bacteria and Viruses

Bacteria and viruses can causes diseases. Infectious diseases are generally passed from person to person through physical contact. They can also contaminate some surfaces, and be passed to another person coming in contact with those surfaces.

Some bacteria and viruses circulate through indoor ventilation systems, particularly if there is a moisture problem in the system. They can also circulate from humidifiers that are not clean.

Strategies That Reduce Growth and Buildup of Biological Pollutants

Control the Moisture (between 30-50 percent humidity)

The amount of moisture, relative humidity, that the air in your home can hold depends on the temperature of the air. As the temperature goes down, the air is able to hold less moisture.

  1. Install and use exhaust fans that are vented to the outdoors in kitchens and bathrooms and vent clothes dryers outdoors. Use of these exhaust fans can eliminate much of the moisture that builds up from everyday activities such as bathing, cooking, laundry, etc.
  2. Ventilate the attic and crawl spaces to prevent moisture build-up. Keep humidity level below 50% to prevent water condensation on building materials.
  3. If using cool mist or ultrasonic humidifiers, clean appliances according to manufacturer's instructions and refill with fresh water daily.
  4. Evaporation trays in air conditioners, dehumidifiers and refrigerators should also be cleaned frequently and thoroughly.
  5. Thoroughly clean and dry water damaged carpets and building materials (within 24 hours if possible) or consider removal and replacement.

Keep the House Clean

  1. Clean hard surfaces with a soap or detergent solution regularly. If mold is found growing on hard surfaces or there is a risk of contamination from disease organisms, surfaces can be cleaned with a disinfectant. Disinfectants are chemical formulations that kill organisms. Chlorine bleach is a common disinfectant.
  2. Vacuum the house when individuals who are allergic or sensitive are not in the house. Since vacuuming will stir up some dust, anyone allergic to the allergens in dust should avoid the newly vacuumed area for 30 minutes.
  3. Use vacuums with high efficiency filters (HEPA filter - High Efficiency Particulate Air filter). Use a vacuum with special allergen-control features if possible. Change or empty dirt collection bag before it gets half full.
  4. A central vacuum system with the motor, suction blower, and filter bag located in the basement or garage is the most desirable system, since there is no air discharge in the room being vacuumed.
  5. Use allergen-proof mattress encasements, wash bedding in hot (130 degrees F) water, and avoid room furnishings that accumulate dust, especially if they cannot be washed in hot water.
  6. Minimize the amount of carpet in living areas. Vinyl, tile or wood flooring and area rugs are better choices than wall-to-wall carpeting.
  7. Place mats or small area rugs in front of all entry ways to collect dirt, dust, pollen and other allergens brought in from the outside and help prevent them from spreading to other areas. Regularly shake these rugs outdoors, away from open windows. Launder them weekly to remove dust/dust mites, animal dander, pollen and mold spores.
  8. Thoroughly vacuum carpets, pillows, draperies and upholstered furniture at least once a week (more often during allergy season or if there are pets in the household). Vacuum against the carpet's nap, going over each area at least six to eight times.
  9. Clean vinyl or ceramic tile floors weekly, using a floor cleaner or a non-abrasive, all-purpose cleaner. To avoid a cloudy floor, rinse it well after each cleaning or use a no-rinse product.
  10. Wood floors should be vacuumed first to remove the surface dirt, then cleaned with a wood floor cleaner.

Air-cleaning Devices

Cleaners that are part of the whole-house air-handling system are probably more effective in lowering the level of contaminants throughout the house than are smaller cleaners. Consumers should carefully assess any air cleaner being considered before purchasing.

Duct Cleaning

There is currently no conclusive proof that air-duct cleaning is effective in reducing or preventing indoor air problems. Duct cleaning, by a professional, may be appropriate if: debris in the ducts, infested with vermin, visible mold, fungi, or other biological contaminants, particles spewing from the registers. Other maintenance on the ductwork may help reduce or prevent contamination problems. Water leaks into ducts should be stopped to prevent mold and bacteria growth. Insulation of the ductwork can reduce condensation problems in ducts that result in contaminant growth. Changing the system's filters regularly or switching to a high efficiency filter may reduce particle contamination.

(Prepared by Janie L. Harris, Extension Housing and Environment Specialist, 1999.)