If your sinuses haven't already alerted you, allergy season is upon us — and it's earlier and stronger than expected.

Spring weather is spreading quickly across the central part of the country, according to the USA National Phenology Network, which tracks climate and seasonal changes and data. Compared to long-term average from 1991 to 2020, Denver is 6 days early, Chicago is 15 days early and Detroit is 23 days early, the network says.

The same was true for last year's allergy season, when several regions of the U.S. experienced springtime conditions weeks early, forcing spring allergy sufferers to deal with symptoms sooner and longer than usual.

Researchers predict these aren't outlier years, pointing to climate change as responsible for worsening allergy season. 

This past winter was the warmest on record across the continental U.S. Fewer days below freezing meant plants were able to bloom earlier and longer.

"Pollen seasons are starting earlier and getting worse with more pollen in the air," William Anderegg, associate professor at the University of Utah, told CBS News, pointing to heat as "one of the biggest drivers."

Between 1990 and 2018, there was a 21% increase in pollen, according to a recent study authored by Anderegg.

Here's what else to know about pollen season this year:

Pollen count for spring 2024

Dr. Rachna Shah, an allergist and director of the Loyola Medicine Allergy Count, told the Associated Press she usually starts looking at pollen counts in the Chicago area in April. But she peeked at her data in mid-February and saw tree pollen was already at a "moderate" level.

"This season has been so nuts," Shah said. "Granted, it was a pretty mild winter, but I didn't expect it to be so early."

Do certain cities have it worse?

According to the Asthma and Allergy Foundation of America's annual ranking, the top five most challenging cities to live in if you have allergies this year are: Wichita, Kansas; Virginia Beach, Virginia; Greenville, South Carolina; Dallas; and Oklahoma City.

This is based on over-the-counter medicine use, pollen counts and the number of available allergy specialists. 

Where does pollen come from?

Pollen is released by trees, grasses and weeds, explains Dr. Neil Parikh, allergist and immunologist with Dignity Health Mercy Medical Group.

"So those are found outside the home, but that pollen can also come inside the home," he told CBS News Sacramento. "Understand that when you go outside and you breathe in that air, you're exposed to the pollen. So the longer it stays on your body, in your nose and your eyes and your sinuses, the more likely you're going to react and feel bad from them."

For that reason, if you're outside with high pollen and suffer from allergies, Parikh suggests a few steps after coming inside, including taking a shower, changing your clothes and doing a sinus rinse with saline water. 

HEPA air purifiers can also help remove the pollen that comes from outside to inside your home, he says.

Can seasonal allergies cause fever, coughing, headaches, sore throat?

There are several allergy symptoms to be aware of, Parikh says, including: 

• Nasal congestion
• Runny nose
• Sneezing
• Itchy eyes
• Coughing

The Cleveland Clinic says allergies can cause a sore throat due to postnasal drip, which is when discharge from your nose runs down the back of your throat.

Allergens can also cause sinus headache even if you have no other allergy symptoms, according to the American College of Allergy, Asthma and Immunology. This is caused by swelling in the sinuses that blocks the openings, prevents drainage and causes pressure to build up.

Fever, however, is not a symptom of allergies.

"If you're experiencing a runny or stuffy nose and a fever, you may have a sinus infection. Sinus infections are caused by bacteria or viruses, not by allergies," the association notes.

In:
• Allergies

Sara Moniuszko

Sara Moniuszko is a health and lifestyle reporter at CBSNews.com. Previously, she wrote for USA Today, where she was selected to help launch the newspaper's wellness vertical. She now covers breaking and trending news for CBS News' HealthWatch.

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