Kinds of Allergen

Sneezing, itchy eyes, congestion -- these symptoms are all too familiar for the millions who suffer with allergies. In fact, almost 25 million children and adults were diagnosed with hay fever in a recent year, according to the Center for Disease Control and Prevention's 2009 National Health Interview Survey. For some, allergies are more than a springtime nuisance. Many allergens are present both indoors and out all year round, leading to a decreased quality of life for some -- and potential life-threatening reactions for others.

Pollen

• Perhaps the most well-known allergen, pollen wreaks havoc on the respiratory systems of millions of Americans each year. Pollen is transported by insects to other plants, but it can also fill the air during windy periods. Seasonal allergies may occur in the spring when pollen from trees, such as elm, hickory, oak and maple, are likely culprits. Grasses are more likely to cause allergies in the beginning of summer, and ragweed pollen causes trouble primarily in late summer and early fall.

Pets

• Furry four-legged friends produce allergens that bother many people. The dander, or dead skin, that pets shed invades homes, covering floors and furniture. Dogs in particular love to greet their owners with a lick on the face, but even canine saliva can produce an allergic reaction. Urine from rodents and guinea pigs is also an allergen.

Food

• Wheat, eggs, fish, peanuts and other common foods cause no problems for most people. For those with food allergies, however, eating one of these foods could be a matter of life and death. Other top food allergens are shellfish, tree nuts, milk and soy, according to the Food Allergy and Anaphylaxis Network. These account for 90 percent of all food allergies in the United States.

Cigarette Smoke

• Plenty of health organizations have blasted the message that smoking cigarettes not only hurts smokers but also those around them. The toxic chemicals released into the air during a smoking session make cigarette smoke a major allergen. It puts those breathing it at increased risk for allergies and several other conditions affecting the respiratory system, such as sinusitis, bronchitis, respiratory infections and asthma.

Roaches and Mites

• Cockroaches are among the most common allergen sources in American households, according to the National Institute of Environmental Health Sciences. These pests leave saliva and droppings behind, both of which are major allergens. Roaches are particularly hazardous to those with asthma. Dust mites also cause allergies. These tiny creatures feed on dead human skin cells and make their homes in bedding, carpets, furniture and curtains.