More than 300 New Yorkers visited local emergency rooms on Wednesday due to asthma symptoms, according to the latest data from the city's health department. That's almost double the count from the previous day.

The ER visits spiked along with the concentration of airborne pollution spirited south from Canadian wildfires this week. The Air Quality Index, a measure of air pollution, peaked at "hazardous" levels on Wednesday afternoon. Exposure to the polluted air can be especially dangerous for older adults, children and people with pre-existing health conditions like asthma.

Asthma-related emergency visits are now the highest they've been since the peak of tree pollen season in late April.

“While we are seeing higher than usual asthma-related visits to the emergency department, these visits and calls are still in the low hundreds and our hospital and health care systems are fully able to respond to patients. This is an important reminder to stay indoors as much as possible, with the windows closed. If you must be outside, a high-quality mask may help.”

As of Thursday afternoon, air quality in the New York metro area had improved, but it was still at "unhealthy" levels, according to the U.S. Environmental Protection Agency. New York and New Jersey health officials recommend that people stay inside as much as possible, mask when they're outdoors and run air purifiers if they have them.