Outdoor activities are now safe in the Philadelphia region again after a smoky couple of days last week, but the threat remains.

Starting July 15, wildfire smoke from Minnesota and Canada enveloped the area, bringing hazy skies and hazardous air particles. The Air Quality Index in Philadelphia was in the 200s range Thursday and Friday, before gradually declining to moderate levels by Monday morning. As of 10 a.m. Monday, the air quality index was at the healthiest level with 45.

While the smoke has mostly cleared in the northeastern United States, hundreds of wildfires continue to burn in Canada and the smoke is expected to impact the Great Lakes region over the next few days, according to the National Weather Service.

Dr. Eric Balaban, a pulmonary and critical care fellow at the Temple Lung Center, told the Local that these severe environmental events are becoming more frequent. Wildfire smoke brought poor air quality and hazy skies to Philadelphia in the last three summers.

Climate change is increasing the risk of wildfires by driving hotter and drier conditions, according to NASA. High winds linked to climate change could carry smoke caused by wildfires down south more often.

“I see days like [last Thursday] as being sharp reminders that this subtle issue is becoming less and less subtle,” Balaban said. “It will be an increasingly frequent reminder that we need to take care of the environment.”

Many areas of Pennsylvania reached “very unhealthy” (purple) and “hazardous” (maroon) levels last Thursday evening. The Pennsylvania Department of Environmental Protection issued a statewide code purple air quality alert for particulate matter for last Friday, meaning everyone needed to cut back on outdoor activities.

Similarly, the Philadelphia Department of Public Health issued a code purple air quality alert for early last Friday as health officials urged residents to stay indoors, close all windows and doors, and set their air conditioning to recirculate mode. Officials also advised people to avoid activities that produce more particulate matter, such as frying food and using wood-burning stoves.

City officials suspended last Friday’s trash and recycling collections to protect workers from the unhealthy air and encouraged outdoor workers to take frequent breaks and consider wearing masks when needed. Officials said collections would resume Saturday but by Monday they were not completely caught up.

The air quality in Philadelphia last week was a public health risk for everyone, including people with no existing conditions, Balaban said. What makes wildfire smoke particularly hazardous is that it becomes more toxic as it travels, carrying both smoke from the trees and existing air pollutants in its path, he said.

Half of Canada’s wildfires, such as the current ones in northern Ontario, burn in large, remote areas that are difficult to reach, Michael Flannigan, an expert on fire management at Thompson Rivers University in British Columbia, told The New York Times. They are often accessibly only by plane, lack roads for firefighters and are either unpopulated or inhabited by small Indigenous communities.

One pollutant carried by wildfire smoke is ground-level ozone, also known as smog. It forms when sunlight hits nitrogen oxide and volatile organics (emissions from cars and many other sources), and the process accelerates in extreme heat. Philadelphia experienced a heat wave last week, with temperatures last Thursday reaching a high of 96 degrees.

The pollutant can cause shortness of breath, chest pain, nausea or dizziness. People experiencing symptoms should seek immediate medical care. It can exacerbate illnesses such as asthma, emphysema and Chronic Obstructive Pulmonary Disease.

Those with underlying health conditions should have their providers’ contact information, extra medications and inhalers on hand, Balaban said. He suggested people improve air quality in their homes by replacing air-conditioning filters.

The smoke also contained particulate matter, known as PM 2.5, and ultrafine particles, both of which can penetrate deep into the lungs and cause inflammation and irritation deep in the airways, Balaban said. It can cross from the lungs and into the bloodstream, and the effects can be particularly dangerous for people with asthma and respiratory health conditions.

Public and environmental health are “inescapably tied” to each other, Balaban said. Researchers have linked chronic exposure to pollution with cancer, heart attacks and other severe effects. The threats are projected to worsen if people don’t take steps to combat the effects of climate change, he said.

“Medicine can help. But when we talk about the conditions that prime the weather to be polluted and to be dangerously warm, like what we’re seeing, I don’t have a vaccine for that,” he said.