As 90 percent of Americans spend their time indoors, their exposure to indoor allergens and airborne particles is high. Researchers are now saying that this could lead to respiratory conditions down the line.

While such as vitamin C might curb some of the symptoms for allergy sufferers, a new study reports it may be what’s around their house that’s causing the problem.

Allergy experts at the American College of Allergy, Asthma and Immunology’s annual meeting said that damp environments and poorly maintained air conditioning and heating systems may be detrimental to indoor air quality.

“A home’s heating and air-conditioning (HVAC) system, if poorly maintained, can become a major source of microbial allergens,” said building scientist Doug Garrett.

Damp environments can also cause mold and other allergens to grow in carpets, which is why it may be a good idea to shampoo them every once in a while if you suffer from indoor allergies. Dust mites may also hide in carpets and may be harder to remove.