The guidelines for staying healthy while big fires burn have long been simple: stay inside and, when possible, use air filters to capture those dangerous microparticles. But the coronavirus pandemic has caused shortages of key air-quality supplies, and is forcing some difficult tradeoffs on indoor air safety.

Both fine smoke particles and the coronavirus can be captured by a class of air filters known as MERV-13, which can be installed in many existing heating and air conditioning systems. But because these filters weren’t in wide use before the pandemic, a sudden surge in demand has created ongoing MERV-13 shortages.

Mike Gallagher, president of HVAC contractor Western Allied, believes that’s going to catch commercial building managers by surprise once the current wave of fires dies down.

“Once the smoke clears, it smells okay outside, but you walk into the building and it smells like smoke. That’s when they realize they need new filters,” says Gallagher. But with waiting lists for MERV-13 filters as long as two months, Gallagher expects many buildings in smoke-affected areas will be forced to temporarily resort to MERV-8 filters, which are not capable of clearing the coronavirus from the air. That could increase the infection risk in shared spaces including offices, restaurants, and movie theaters.

The coronavirus is forcing a second difficult choice as the fires rage: whether to let in outside air to reduce the risk of infections, or seal buildings up tight to keep out smoke particles.

“For [protection against] COVID, you want to get the [outside air] ventilation rate as high as possible,” says HVAC veteran Tom Javins. “But with wildfire smoke, you want to have the ventilation rate as low as possible. Because the pollutant is in the outside air, not the inside air.”

Javins sits on a committee of the American Society of Heating Refrigerating, and Air-conditioning Engineers (ASHRAE), which is developing best practices for heating and ventilation under wildfire conditions. Though the double-bind of the coronavirus and smoke is challenging, his bigger concern is widespread indifference: “Most [commercial] building managers don’t do anything” to adjust for dangerous outside air quality, he says, and he frequently finds that the vents controlling outside air flow for big buildings have broken down entirely. Most often these vents, known as air dampers, are stuck closed, helping keep out fire particles but increasing COVID-19 risk for occupants.

A separate set of challenges comes with trying to protect private homes from wildfire smoke. According to Sarah Coefield, an air quality specialist with the Missoula, Mont. county government, many homes in the west and Pacific Northwest don’t have central HVAC systems capable of filtering out smoke particles. That leaves them reliant on portable air filtration devices, which are often effective but can also be hard to find right now due to coronavirus-driven demand.

Even when they’re available, portable air filters can be expensive, easily costing up to several hundred dollars for a device big enough to clean the air in a single room. That highlights a larger issue: As seen with the coronavirus, the well-off are more able to protect themselves from the health impacts of wildfires. On top of the cost of filters, lower-end homes or apartments may have more leaks around windows and doors that let in contaminants. And, again as with COVID-19, not all workers are equally able to protect themselves—farmworkers, for instance, can’t work from home to get away from smoke.

That’s one reason that, despite his professional background, Javins is uneasy with focusing on better air filtration to combat the health risks of wildfire pollutants. “We’re talking about how to deal with the symptoms,” he says, “and we’re not really talking about how to deal with the problem”—the accelerating rate and intensity of wildfires. Experts blame the wildfire surge on a combination of poor forest management and human-caused climate change. Those are problems no air filter can solve.