Molds, a type of tiny fungus that are related to mushrooms and mildew cause allergies in some people. They are so small that you can only see them with a microscope. Like pollens, mold spores — or the seeds of molds — drift on currents of wind. Each spore can produce new mold, which, in turn, can produce millions of new mold spores. In some areas, there are so many mold spores in the air, they outnumber the millions of pollen grains. Fortunately, only a few of the many types of molds cause nasal allergy symptoms.
Weather conditions, such as rain, wind, and temperature, affect molds. In many warm areas of the US, such as the South and the West Coast, outdoor molds are present year-round. Molds can live after the first hard frost, and some can even grow at subfreezing temperatures. However, most become dormant in cold weather. A blanketing of snow dramatically reduces the mold counts, but it doesn't kill the molds. Molds growing indoors, such as in a damp basement or bathroom, can cause year-round allergy symptoms even in very cold climates.
Dust Mite Allergies
Under the microscope, dust mites look like giant, humpbacked spiders. They're actually so tiny — about 1/3 mm — they're invisible to the naked eye. Mites feed on the dead skin cells you shed each day.
If you're allergic to dust mites, it's not the creatures themselves that cause an allergic reaction but their waste products. The millions of mites in your home live for about a month. Each mite produces 10 to 20 waste particles every day. These droppings break down into a fine powder that sticks to all kinds of materials. When they're breathed in, they trigger allergy symptoms in sensitive people.
For some people, the proteins in the droppings of cockroaches trigger nasal allergies. This is a special problem in older urban buildings and places that aren't well cleaned. Many children who are continually exposed to cockroaches early in life develop allergies. Some develop asthma, and some develop both allergies and asthma.