Here comes spring, a potentially uncomfortable season for an estimated 81 million people in the United States who are allergic to the onslaught of pollen from trees, grasses and weeds this time of year, according to the Asthma and Allergy Foundation of America.

Allergies result from the immune system’s reaction to something that for most people is harmless — for example, pet dander, nuts or, in the spring, pollen. Symptoms of allergic rhinitis (sometimes referred to as hay fever) may include itchy and watery eyes, nasal congestion, frequent sneezing and a sore throat.

To get ahead of likely symptoms, doctors sometimes suggest starting regular use of an over-the-counter nasal spray a week or two before allergy season strikes in your area. Also, check with your doctor about whether using an antihistamine or decongestant once the season reaches full swing would be safe for you, taking into account other health issues you may have, such as hypertension or heart problems.

Some allergy sufferers choose immunotherapy (a series of allergy shots) to reduce their body’s reaction to allergens — substances that cause allergic reactions. Beyond medical approaches, avoidance is considered key to keeping symptoms of allergic rhinitis at bay.

You can minimize exposure to pollen by keeping windows closed; washing any clothes worn outside and taking showers after being outdoors; having a clean, good-quality filter on your home’s heating and air-conditioning system; and using an air purifier in your residence.

Springtime allergies may last longer than they have historically, say experts in both weather and medical realms, thanks to a warmer climate, which lengthens the pollen season.

This article is part of The Post’s “Big Number” series, which takes a brief look at the statistical aspect of health issues. Additional information and relevant research are available through the hyperlinks.