Allergies are common among Americans: Researchers believe nasal allergies alone affect about 50 million of us, and their incidence is increasing. While some symptoms, like the sniffing and sneezing that usually accompany seasonal allergies, are annoying and may interfere with day-to-day activities, other allergies such as seafood or peanuts can be fatal.

According to the American College of Allergy, Asthma & Immunology (ACAAI), allergies are the sixth leading cause of chronic illness in the United States. Food allergies, which are among the deadliest, send more than 200,000 people to the emergency room every year.

Here's a look at the most common causes of allergies:

Pollen. Trees begin pollinating with the first breath of spring, with oak trees topping the list of culprits. Evergreens, birch, sycamores, and poplars can all cause sniffles, sneezing, nasal congestion, and watery, itchy eyes. Millions of people are plagued by grass pollen from late spring into early fall. Bermuda grass and timothy grass top the list of the worst offenders.

Fall allergies are usually caused by weeds, with ragweed sitting on top of the list of offenders. It's almost impossible to avoid, since 17 different species are found across the U.S.

Mold. Mold can be found outdoors or in your house, and is a year-round allergen. Inside, it thrives in damp spots, such as bathrooms and basements, and it can also be found in refrigerator and air conditioner drip pans. Outside, it can thrive in piles of leaves and clogged drains. Mold spores trigger sneezing, nasal congestion, and itchy, watery eyes as well as asthma-like symptoms.

Dust mites. These tiny critters live in your carpets, bedding, toys, and in the fur of your pets, and about 20 million Americans are allergic to them and their waste products, which contain a protein that triggers allergic reactions. In addition to sneezing and a runny nose, many people allergic to dust mites also show signs of asthma, such as wheezing and difficulty breathing.

Although certain foods might give you diarrhea or gas, these are food intolerances, not true allergies. True allergies can cause reactions similar to pollen, such as itchy eyes, but they can also cause a reaction called anaphylaxis, which interferes with breathing and can be deadly. A trip to the emergency room or quick access to an injection of epinephrine (EpiPen) is necessary.

Peanuts. More than three million Americans are allergic to peanuts, and a reaction can be triggered by a minuscule amount. Reactions can be severe and cause the highest numbers of anaphylactic shock compared to other allergies. Although peanut allergies usually last a lifetime, studies show that about 20 percent of children will outgrow them. Since peanuts are legumes, those with peanut allergies have a higher risk of being allergic to other legumes, including soy. Other studies have indicated that up to 40 percent of people who are allergic to peanuts are also allergic to tree nuts.

Shellfish. More than 2 percent of adults have allergies to shellfish, which can include shrimp, crab, lobster, oysters, clams, and mussels. Shrimp, lobster, and crab cause the greatest number of reactions, according to the ACAAI, and many people who are allergic to shrimp, lobster, and crab can eat mollusks (oysters, clams, mussels, and scallops). Unlike many other allergies, shellfish allergies usually develop later in life. Victims often develop anaphylaxis, which demands immediate treatment.

Insect bites. Bites or stings from insects — honeybees, wasps, yellow jackets, hornets, and fire ants — leave proteins in the skin from the venom that trigger allergic reactions in some people. Although swelling, itching, and pain around the bite are common, bites can also trigger anaphylaxis, which can include hives swelling in the throat, and difficulty breathing, and is life-threatening.

Treat seasonal allergies by taking over-the-counter antihistamines, such as Allegra, Claritin, or Zyrtec, 30 minutes before going outside. At home, keep windows and doors closed, and run air conditioning or air cleaning systems with high-efficiency filters (HEPA) filters to remove allergens from indoor air.

For indoor allergens, dust frequently, vacuum carpets and upholstery on a regular basis, use "dust proof" bedding for dust mites, and wash bed linens in hot water. Keep mold at bay by using dehumidifiers and fans to reduce moisture, and by fixing plumbing leaks as quickly as possible.

For natural help in combatting allergies, try butterbur, quercetin, and fish oil. Eating honey made from local flowers and plants may help "immunize" your body against the pollen in your area. A study published in the International Archives of Allergy and Immunology found that honey decreased the symptoms of allergy to birch pollen by 60 percent, and reduced the need for antihistamines by half.

If you know you have serious reactions to certain foods or insect stings, and have had anaphylactic shock in the past, carry epinephrine (EpiPen). In reaction to the high price of EpiPens (about $600), CVS is selling a generic version for $109.99 — about a sixth of the price.