(NEW YORK) — As temperatures begin to warm up, announcing the arrival of spring, it also means the arrival of allergy season for millions of Americans.

Health departments across the U.S. have started to warn that high pollen levels could mean exacerbated symptoms including itchy and watery eyes; runny noses; sneezing; hives; and coughing.

According to the U.S. Department of Agriculture (USDA), recent research has shown that pollen seasons start 20 days earlier and are 10 days longer than they were in 1990.

Allergists told ABC News a mix of climate change and more carbon emissions in the atmosphere have led to plants in many areas having a longer growing season and, therefore, higher pollen levels.

In fact, the USDA said allergy seasons feature 21% more pollen than three decades ago.

“Each earlier, longer growing season just leads to more time for exposure to those things which patient may react to,” Dr. Arveen Bhasin, an allergist-immunologist at Mayo Clinic, told ABC News.

Here’s what people with seasonal allergies can do to help handle their symptoms and get some relief:

Why do seasonal allergies occur?
Allergies occur when the immune system views food, medicine, things in the environment or something else as harmful and overreacts.

“Seasonal allergies occur when your immune system, which is normally designed to fight unwanted things like germs, becomes hyperactive in an abnormal way against anything, in this case airborne allergens,” Dr. Purvi Parikh, an allergist and immunologist at NYU Langone Health, told ABC News. “Pollen is the biggest culprit for season allergies,” which can come from plants such as trees, grasses and weeds.

The immune system then releases chemical compounds, such as histamine, which causes those hallmark symptoms of allergies.

Reactions can range from mildly annoying symptoms to life-threatening reactions including anaphylactic shock, which is when blood pressure drops suddenly and the organs can’t get enough oxygen.

In 2021, approximately 81 million people in the U.S. were diagnosed with seasonal allergies, otherwise known as hay fever, according to the Asthma and Allergy Foundation of America (AAFA).

How long do seasonal allergies last and are they getting worse?
Typically, spring allergies begin in March and last until the early summer while fall allergies begin in late August and last through autumn.

However, a report released last year from the AAFA found cities throughout the country — from Scranton, Pennsylvania, to San Antonio, Texas — are reporting worsening seasonal allergies.

Additionally, research has suggested that allergy seasons were getting longer and worse.

“Because our planet is warming up, it’s taking longer to get that first frost in the fall and so, there’s just longer growing seasons, and then we’re seeing the pollen in the spring start to come up earlier,” Bhasin said.

Parikh said that because more carbon dioxide has been released into the air, and plants feed off of carbon dioxide, this has released more pollen into the air, too.

“It’s a trend that we’ve been seeing over the past decade and [allergy seasons have] been gradually getting worse,” she said. “This year was especially bad. Usually we start seeing patients mid-March and this year we started seeing patients come in February with symptoms, and I think it’s because we had such a warm winter.”

The COVID-19 pandemic also had an effect on allergies. According to the AAFA report, few people reported pollen allergies in 2020 because people were inside due to workplace and school closures and stay-at-home orders.

“These trends continued in spring 2021,” according to the report. “But by fall 2021, the number of people affected by seasonal allergies rose to pre-pandemic levels. Experts believe this jump was due to fewer COVID-19 restrictions and more people going out, likely because of the introduction of COVID-19 vaccines.”

How can I treat my seasonal allergies?
“First and foremost is trying to identify what specifically patients are reactive to, so then we can talk about trying to control exposures or avoidance measures,” Bhasin said. “A lot of times that’s not effective for outdoor pollen allergies, but whatever we can do to try to mitigate some symptom exposure helps.”

She said there are a number of over-the-counter medications that people can try as well as nasal sprays and rinses.

Some are tailored to relieve symptoms while others are used to prevent symptoms. Additionally, certain medications work for certain symptoms.

Parikh sad because some patients can have more severe reactions to allergens than others, not all medications will be effective for all patients.

If people are not sure if their symptoms are linked to allergies, or they’re having trouble controlling their allergies, they should visit an allergist or immunologist.

Bhasin also recommends being proactive rather than reactive when it comes to treating seasonal allergies.

“By that, I mean start your medications, a couple of weeks before the season is anticipated to start,” she said. “So, take your antihistamine, take your nasal spray, start those a couple of weeks before so that, when the pollen hits, you’ve already got some protective measures within you.”

How do I distinguish season allergies from COVID-19?
Although COVID-19 and seasonal allergies share many symptoms, there are some differences.

For example, itchy, watery eyes and sneezing are common symptoms of seasonal allergies but rarely are symptoms of COVID-19.

However, symptoms such as fever, headaches or loss of taste and smell are more likely due to a respiratory illness like COVID-19.

“I think if you’re concerned about COVID, get a COVID test and see,” Bhasin said. “But for the most part, patients will say, ‘Hey, listen, every spring or every summer, I seem to get these symptoms, and I’m miserable for a few years."”

She continued, “So there’s some historical pattern for allergies, whereas COVID, you know, he’s going to tend to come out of nowhere, if you will.”

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