A classically harsh August for weed allergies has given way, so far, to a milder September.

But — sniff, sniff — allergy season might not be over yet.

Drawing on research showing that climate change has lengthened the annual ragweed season by 18 to 21 days in Minnesota, state Health Department officials are warning allergy sufferers to remain on alert.

“Eighteen to 21 days? You’re talking three weeks longer!” said Wendy Brunner, a program manager and epidemiologist for the department’s asthma program, and an allergy sufferer herself. “That’s a much longer time.”

An influential 2011 study by Lewis Ziska of the U.S. Department of Agriculture documented the extended ragweed season by reviewing pollen counts in the central United States from 1995 to 2009 and adjusting results based on changing frost and weather patterns. A 2016 update to his research examined the counts through 2015, and found that the growth in ragweed season was more dramatic among northern communities.

Pollen count and weather data over the two decades showed the season had increased by only one day in Oklahoma City, but it had increased by 18 days in Minneapolis, and by 24 days or more in the Canadian cities of Winnipeg and Saskatoon.

The ragweed season in Minneapolis appears to still start in mid- to late July, but then to extend later into September and October, according to Ziska’s data.

People with allergies should limit outdoor activities, wear hats and sunglasses to block out pollen, keep windows closed, and immediately wash clothes worn outside, the Health Department advised.

The department also has created a historical pollen count page so people can accurately plan and prepare for changing allergy seasons.

The state data from 1995 to 2016 do not show an increase in the length of the weed allergy season. But Brunner said it reflects raw data, which doesn’t account for weather pattern changes from year to year as Ziska did. The state data also show overall weed allergy levels, whereas Ziska’s study specifically focused on ragweed.

The most recent year in the state data, 2016, was unusual for its intensity. Weed pollen counts were measured in Minneapolis in the high or very high ranges for 65 days that season. No other season since 2010 had more than 45 such days.