An allergy is a reaction to a substance called an allergen. Common allergens include:

• Wind-borne pollen (from grass, trees, ragweed)

• Mold

• Dust mites

• Furry and feathered animals

• Pests, such as cockroaches or rodents in a home or other building

Normally allergens are harmless. But when a person has allergies, the body thinks these allergens are harmful. The body then attacks allergens with antibodies. Allergy antibodies are attached to special cells called mast cells. Allergens stick to the antibodies. This makes the mast cells release histamine and other chemicals. This is an allergic reaction. The chemicals irritate nearby nasal tissue. This causes nasal allergy symptoms. When this happens in the breathing tubes of the lungs, it can cause asthma symptoms such as cough and wheeze.

Allergies can cause nasal tissue to swell. This makes the air passages smaller. The nose may feel stuffed up or itchy. The nose may also make extra mucus. This can plug the nasal passages or drip out of the nose. Mucus can drip down the back of the throat (postnasal drip) as well. Sinus tissue can swell. This may cause pain and headache. Common allergy symptoms include:

• Runny nose with clear, watery discharge

• Stuffy nose (nasal congestion)

• Drainage down your throat (postnasal drip)

• Sneezing

• Red, watery eyes

• Itchy nose, eyes, ears, and throat

• Plugged-up ears (ear congestion)

• Sore throat

• Coughing

• Sinus pain and swelling

• Headache

Other health problems can cause symptoms like those of nasal allergies. These include:

• Nonallergic rhinitis and viruses such as colds

• Irritants and pollutants such as strong odors or smoke

• Common home cleaning products, especially if they have strong odors

• Certain medicines

• Changes in the weather

Your healthcare provider will evaluate you to find the cause of your symptoms. Then they'll advise treatment. If your symptoms are due to nasal allergies, your provider may prescribe nasal steroid sprays or antihistamines taken by mouth (oral) to help reduce symptoms. Staying away from the allergen will also be suggested. You may also be referred to an allergist. 

Pollen comes from plants, including trees, grass, and weeds. Plants reproduce by moving tiny grains of pollen from plant to plant. Some pollen is carried by bees. And some is blown by the wind. It’s the wind-blown pollen that causes nasal allergies. The amount of pollen in the air changes from season to season.

Dust mites are tiny bugs too small to see. They can live in mattresses, blankets, stuffed toys, and carpets. Their droppings are a common indoor cause of nasal allergies. Dust mites are normal. They're not an infestation and they don't bite.

Mold loves dark, damp areas, both indoors and outdoors. It tends to grow in bathrooms, basements, refrigerators, and in the soil of houseplants. Mold reproduces by sending tiny grains called spores into the air. If these spores are breathed in, they can cause a nasal allergic reaction. Mold is also present in decaying vegetable matter. For instance, in piles of wet leaves in a yard.

Pets are common causes of nasal allergies. This includes cats, dogs, birds, horses, and rabbits. Flakes of skin (dander), saliva left on fur when an animal cleans itself, urine in litter boxes and cages, and feathers can all cause nasal allergies. Pests such as rodents can also be a major cause of allergies. This is especially the case in urban settings. They may not even be in your own apartment.

Irritants don’t cause nasal allergies. But they can make allergy symptoms worse. Examples of irritants include:

• Cigarette smoke
• Aerosol from e-cigarettes and other vaping devices
• Perfume
• Aerosol sprays
• Smoke from wood stoves or fireplaces
• Air pollution
• Car exhaust
• Strong odors such as from cleaning products

Fluid may be trapped in the sinuses. Bacteria may grow in trapped fluid. This causes sinus infection (sinusitis) and pain. Sometimes the sinus pain can spread to the jaw and be mistaken for dental problems.

Allergens irritate your eyes, including the lining of the conjunctiva. This is a common eye infection, especially among children. It causes eyes to become red, itchy, puffy, and watery. Some forms of conjunctivitis are caused by viruses and are highly contagious. These viruses can spread quickly, especially in schools. But conjunctivitis caused by allergens isn't contagious.

The eustachian tube connects the middle ear to nasal passages. Allergies can block this tube, and make the ears feel plugged. Fluid may also build up, leading to an ear infection (otitis media).

Allergies cause nasal passages to swell. Constant swelling can lead to formation of a sac that comes from the sinuses called a polyp. Polyps can grow large enough to block nasal passages. Nasal polyps (unlike other polyps) don't cause cancer. But they can cause mild pain and a decreased sense of smell and taste.

Asthma is inflammation and swelling of the air passages in the lungs. The symptoms are wheezing, shortness of breath, coughing, and chest tightness. Allergies, including nasal allergies, are common in people with asthma.