PORTLAND, Ore. (KOIN) – If it seems like allergy season has you sneezing earlier each year, it’s not just you. A new study shows allergy season is starting earlier and lasting longer in cities around the United States, including four in Oregon. 

Portland, Eugene, Medford and Bend are among dozens of cities in the Climate Central report published March 7 with allergy seasons that have grown longer in the last 52 years. 

According to the non-profit news organization's report, climate change is altering allergy season, making it longer and more intense. 

Seasonal allergies are caused by pollen and mold spores released throughout the year, but their optimal conditions tend to be when it’s warmer. Mold also requires moisture and organic material to germinate and grow. 

“From warming temperatures and more freeze-free days to increased carbon dioxide in the atmosphere, climate change is making allergy season longer and more intense, with higher amounts of allergens in the air,” the report states. 

The report cites a study published in the Proceedings of the National Academy of Sciences journal that says pollen seasons became longer by an average of 20 days and more intense, with a 21% increase in concentrations, from 1990 to 2018. 

The study measures allergy season based on the length of the growing season, which lasts from the last freezing day in spring to the first freezing day of fall. Using data from the Applied Climate Information System collected since 1970 for 203 U.S. cities, Climate Central found that the freeze-free season is lengthening and on average gives plants more than two weeks to grow, flower and release pollen. 

Scientists say the warming temperatures caused by climate change mean that allergy season arrives sooner in spring and lasts longer into fall. 

Of the more than 200 cities analyzed, 85% of them saw their freeze-free seasons lengthen during the study period. 

In Portland, the growing season increased by 26 days from 1970 to 2022. In Eugene, it increased 38 days, and in Medford, it’s now 63 days longer. Bend was the Oregon city with the greatest increase of 83 days. 

According to Climate Central, since 1970, the freeze-free season lengthened the most in the western United States and the least in the central region. 

Researchers for the study say longer, more intense allergy seasons pose a significant threat to human health. Seasonal allergens not only cause nasal congestion, irritated eyes and sneezing, but can also trigger or aggravate asthma. Mold spore exposure can also result in hospitalization. 

These symptoms can be expensive to treat with medication, especially for people whose allergies require allergy shots.

Climate Central said cutting greenhouse gas emissions is the most meaningful action people can take to slow the rate of global warming and reduce the rate at which allergy seasons are increasing. 

Anyone affected by allergies can take measures to protect themselves, such as checking local air quality reports and allergen forecasts before heading outside, closing windows to keep pollen and spores out of homes, and maintaining indoor humidity between 30-50% to minimize mold growth.  

Researchers at Climate Central say mold allergens are likely similarly affected by a changing climate, but say more research is needed to identify the specific impacts of climate change on outdoor mold.