Here are the most bothersome allergens in the environment, primarily in the home. Allergens indoors tend to be more intense than allergens outdoors because they’re more concentrated and people sleep with them. Experts say the key to reducing allergies indoors is to keep the house clean and remove things that collect dust.

Dust mites

Doctors consider these microscopic pests in the spider family the worst triggers for in-home allergies. Difficult to control, they live in bedding, carpets and any place people leave their favorite food.

• Transmission. Their droppings cause allergic reactions.
• Remedy. Cover surfaces in airtight plastic. The American Lung Assn. recommends washing bedding at least once a week. Vacuum and clean the rug as often as possible. An expensive remedy is to remove the carpeting from the house, at least the bedroom. Dust frequently and keep ventilation pathways clean. Doctors say air cleaners and allergy air filters on furnaces and air conditioners help, but not as much as manufacturers would have you believe. Humidity below 40 percent reduces mites’ numbers, so consider a dehumidifier.

Mold

Mold is a fungus that grows in moist, dark places. It thrives in the moist soil of indoor plants and on their leaves.

• Transmission. The mold spores become airborne and cause allergic reactions.
• Remedy. Dry out the house with a dehumidifier. Clean hard surfaces with household cleaners. The bathroom and kitchen are the worst places for mold.

Pets

Dogs, cats and birds (and any fur-bearing animals) produce allergens in the form of dander (think of it as animal dandruff). The protein in dog and cat spit is the culprit. Residue on feathers can cause allergic reactions.

• Transmission. Once dander is in a home, it can remain for years, even if the pet leaves. Check with a doctor before introducing an animal into your home.
• Remedy. That’s up to the pet owner. If an allergist tracks a childhood allergy to a pet, it’s only a matter of time before the pet has to go, doctors say. The reason is that allergies in children produce prolonged irritation and can lead to asthma. For adults, tolerate a pet as much as you can tolerate the allergies. Otherwise, learn to love fish or reptiles.

Cockroaches

The yuck-bug of the American home triggers allergies. Up to 40 percent of children with allergies are allergic to cockroaches.

• Transmission. It occurs from their droppings, body parts or secretions.
• Remedy. They love to hide in damp, warm places. In an urban area, removing them is virtually impossible without torching the neighborhood. Less drastic measures: Keep the kitchen clean, store food in sealed containers, put away pet food, seal garbage and remove grease from oven surfaces. Powders and traps are effective with those measures. For people with allergies, spray pesticides may be worse than the roaches. When leaving for any period of time, consider putting a roach bomb in the home.

Smoke

Most smoke that’s a problem comes from tobacco products, although smoke from fireplaces and other sources also can irritate respiratory systems. The American Lung Assn. is pushing a program to ban smoking in the home.

• Transmission. Smoke is an irritant that can trigger asthma and allergies. If you smoke indoors, everyone in the home with you might as well be a smoker. The effect is worse for people with asthma or allergies.
• Remedy. Give up smoking, and forbid anyone to smoke in the home.