Air quality alerts still in place as smoke hangs over Philadelphia region | Live updates

With the sun obstructed by haze, Spain's players practiced Thursday ahead of the World Cup final outside in northern New Jersey while air conditions are hazardous because of smoke from Canadian wildfires.

It was not clear how fast-paced a practice Spain held. Media members are only able to observe the first 15 minutes of what was scheduled to be a hourlong session in East Hanover beginning at 11 a.m. EDT.

Officials urged people to stay inside or wear masks outside as air quality reached unhealthy to hazardous levels, meaning it's unhealthy for anyone, regardless of health conditions. Experts expressed concern over holding practice outdoors.

"These are high-level athletes who are moving a lot of air through their lungs during every practice in every game, and really they shouldn't be practicing outside if the air quality levels are at hazardous sort of ranges for wildfire-related air pollution," said Dr. Courtney Howard, an emergency room physician and Global Climate and Health Alliance official. "That's the time to schedule a practice inside. You could put an N95 mask on them, but trying to make sure that everybody's mask is well-fitted, I suspect that's not the best choice. I would go find an air-conditioned indoor facility that's a clean-air shelter."

Messages sent to FIFA and the Spanish Football Association asking whether that was considered or possible were not immediately returned. The smoke is expected to clear the area well before the championship game Sunday in East Rutherford, New Jersey, with kickoff scheduled for 3 p.m.

Smoke from wildfires - which are burning more of North America as Earth warms - attacks nearly every system in the body, killing tens of thousands of people a year, numerous medical studies show.

It attacks the body immediately, spiking asthma cases with increased ambulance runs within hours. Smoke can trigger inflammation in different parts of the body, often attacking a person's weakest points, which can then cascade into different effects of an immune system trying to fight a nasty irritant, doctors and scientists said.

"It's not healthy for anyone to be in the smoke, especially if you're exercising," Harvard School of Public Health environmental health research scientist Mary Johnson said. "You're exchanging more air, so you're being exposed to even more pollutants, and even healthy individuals at some point will have some type of health effect from the exposure to the smoke. So, even though these are healthy, young individuals, it's not a good idea to be exercising in this type of environment."

Scientists have counted at least 1,000 toxins in wildfire smoke, according to Colorado State University environmental toxicologist Luke Montrose.

"If I gave you a list you would recognize some of these as being very bad often times associated with the burning of diesel fuel or cigarette smoke things like formaldehyde or volatile organic compounds," Montrose said. "Just the smoke itself can be bad."

New Jersey Governor Mikie Sherrill says she has directed NJ TRANSIT and the New Jersey Turnpike Authority to make select quantities of masks available to commuters and at rest areas along the Turnpike and Parkway as smoke from Canadian wildfires continues to impact air quality in our region.

Sherrill added that the Port Authority will also be distributing masks.

NJ Transit says travelers can stop by a participating Customer Service location and ask a representative for assistance.

Officials say masks will be available while supplies last.

ABC News' Chief Meteorologist Ginger Zee reports on the Texas storms causing flash flood emergencies and the air quality alerts in the Midwest and Northeast.

ABC News' Chief Meteorologist Ginger Zee tracks timing of smoke conditions

For those who need to work outdoors, are more sensitive to smoke or are in a high-risk group, wearing a mask can help protect you -- as long as it's the right kind of mask.

Kai Chen, an associate professor of epidemiology at Yale School of Public Health, told ABC News that the best type of mask to wear is an N95, which is designed to filter at least 95% of airborne particles.

Chen said wildfire smoke is made up of fine solids, gases and fine particles, the latter of which is the most concerning.

Fine particulate matter, known as PM2.5, is 30 times smaller in diameter than a human hair. Because these particles are too small to be seen with the naked eye, they can easily enter the nose and throat and can travel to the lungs. These tiny particles get trapped in the lungs, and some of the smallest particles can even circulate in the bloodstream, according to the American Lung Association.

PM2.5 can cause both short-term health effects, even for healthy people, including irritation of the eyes, nose and throat; coughing, sneezing and shortness of breath and long-term effects such as worsening of conditions including asthma and heart disease.

"In order to protect yourself from that, we really need powerful filtration ... wearing an N95 mask is key," Chen said. "The normal surgical mask, unfortunately, would not be good enough."

Chen said surgical masks can block the majority of respiratory droplets from a cough or a sneeze but are not effective at filtering PM2.5. Similarly, paper dust masks, cloth masks, wet cloths and bandanas are not effective at filtering out fine particulate matter, he said.

Chen said people should make sure their N95 mask is certified by the National Institute of Occupational Safety and Health. The N95 should have two straps that go around the head and fit over the nose and under the chin, creating a tight seal.

N95 masks will not work properly without a tight seal, so children or people with facial hair might get reduced protection. Most healthy people can wear them safely, but some may notice it takes more effort to breathe due to the snug seal.

If you develop significant shortness of breath, dizziness while wearing an N95 mask, remove it once you are in a safe cleaner-air environment, and seek medical attention if symptoms do not improve.