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A research effort to see how long it takes people to recover from living with hazardous levels of wildfire smoke for seven weeks still hasn’t determined the answer.

Some residents of the western Montana town of Seeley Lake who stayed in the area during the 2017 wildfire season are participating in a University of Montana study of their lung capacity.

Researchers found that people’s lung capacity declined in the first two years.

The percentage of participants whose lung function fell below the lower limit of normal more than doubled one year following the wildfire.

“These findings suggest that wildfire smoke can have long-lasting effects on human health,” researchers wrote. “As wildfires continue to increase both here and globally, understanding the health implications is vital to understanding the respiratory impacts of these events as well as developing public health strategies to mitigate the effects.”

Kaiser Health News reports researchers don’t know how the residents are faring this year because they could not return to Seeley Lake due to the coronavirus pandemic.

People in California, Washington state and Oregon have been struggling for more than a week under some of the most unhealthy air on the planet. The acrid yellow-green smog may linger for days or weeks, scientists and forecasters said.

With wildfires getting larger and more destructive because of climate change and more people living closer to areas that burn, smoke will likely shroud the sky more often in the future.

Smoke can irritate the eyes and lungs and worsen some medical conditions. Health experts warned that young children, adults over 65, pregnant women and people with heart disease, asthma or other respiratory conditions were especially vulnerable.

“The lasting effects of breathing the small particulates in the wildfire smoke can be extremely dangerous,” said Sarah Present, a health officer in Oregon. “It can lead to heart attacks, irregular heart rhythms and even death.”

The region has had a significant increase in visits to emergency rooms due to air quality, officials said Tuesday.

Some parts of central California are not likely to see relief until October, according to Dan Borsum, the incident meteorologist for a fire in Northern California.