Molds are microscopic fungi that can grow on most any wet surface. Mold can affect people in different ways, from minor allergic reactions such as red eyes, skin rash and nasal stuffiness to severe complications like fever and shortness of breath. People with obstructive lung diseases or weakened immune systems may even develop mold infections in their lungs.

A common cause of respiratory infections, airborne bacteria pose a specific threat to infants, the elderly and those with weakened immune or cardiovascular systems, including individuals with asthma, allergies, heart disease and AIDS. Airborne bacteria and viruses also spread serious infectious diseases such as meningitis, whooping cough and pneumonia.

Dust mites are microscopic, eight-legged insects that live within fabric materials such as carpets, drapes, bedding, upholstered furniture and even stuffed animals. Each dust mite produces about 20 fecal droppings per day, and each female lays up to 50 eggs every three weeks. A typical, full-size bed in your home may house more than 2 million asthma-causing dust mites.

Pet dander is not the fur or hair of an animal; rather, it consists of old skin flakes, which are constantly shed. Many people are allergic to pet dander, which causes runny nose, watery eyes and even asthma attacks. Because pet dander is extremely lightweight and tiny, it can stay airborne for hours. Pet dander can also remain in your home for up to six months after the pet itself has been removed.

Many household products, such as paints, varnishes, binders, adhesives, cleaning chemicals and other petroleum-based products, contain organic chemicals. These products can emit gaseous VOCs, which contain a variety of harmful chemicals that can cause eye, nose, and throat irritation, nausea, headaches and damage to the liver, kidney and central nervous system.

Each year, trees, weeds and other plants release tiny powderlike particles called pollen. Carried by the wind, these microscopic particles cause seasonal hay fever and seasonal asthma in over 30 percent of the population. These allergies usually occur in spring and early summer, although in some cases, they can begin in late winter and extend through the summer.