As thick smoke from Canada’s wildfires envelopes wide swaths of the country, sending people indoors, there comes a nagging question for many people: If it’s that bad out there, what about the air inside my home?

Wildfire smoke is very bad for you. The toxic air particles from the smoke can cause coughing and wheezing, headaches, stinging eyes, a scratchy throat, sinus irritation, chest pain, rapid heartbeat and fatigue.

These particles are “much, much smaller than the width of your hair,” said Peter DeCarlo, a professor of Environmental Health and Engineering at Johns Hopkins University. “When you breathe them, they can get deep into your lungs.” People with chronic respiratory illnesses, like asthma, are at serious risk, according to the Centers for Disease Control and Prevention, as are children, older adults and pregnant people.

Experts recommend staying inside and closing windows and doors when the air quality drops. However, wildfire smoke can still enter residential buildings, including through natural ventilation, mechanical ventilation and building facades, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

Here’s what you should know about keeping the air inside safe.

How can I find out what my home’s air quality is?

Unless you have air-quality monitors, you won’t know exactly how healthy (or not) the air is inside your home, but you can keep an eye out on the air outside to inform your decisions.

Monitoring the Air Quality Index, or A.Q.I., will clue you in to what to expect. The air is considered hazardous when the quality index, which measures the density of five pollutants, tops 300. Anything above 150 is considered “unhealthy.” Most weather apps report the A.Q.I., according to Masoud Ghandehari, a professor of urban systems engineering at New York University.

Dr. DeCarlo said that while he loved the idea of everyone’s having an air-quality sensor, the device is an investment that “may give you more understanding of what’s happening, but it’s not going to protect you.”

“If you’re going to invest in something, invest in something that’s going to clean the air,” he said.

Can the outside air enter through my HVAC system?

Generally, it’s not possible for smoky outside air to contaminate the air inside a home through a central air-conditioning system because most of them do not have a dedicated outdoor air intake, said Mike Gallagher, a member of ASHRAE and president emeritus of Western Allied Corporation. But he cautioned that “if there is duct leakage in an attic, for example, then there may be smoke brought in if the attic is ventilated.” And some newer HVAC systems, such as those that prioritize energy efficiency, do have ventilation systems that bring in outside air.

Regardless of which system you have, it’s important to have the right kinds of filters in your HVAC systems, Dr. DeCarlo said, noting that homeowners and renters should look for filters with a high Minimum Efficiency Reporting Values, or MERV, rating.

“The higher the number, the better they are at filtering out,” he said. “Once you get to 10 and higher, you’re going to be able to filter out particles more efficiently in your own air-conditioning system.”

Watch for what Dr. DeCarlo called “pressure drop” in HVAC systems. While a MERV rating of 13 or higher means a filter will take out a majority of concerning particles, older systems might not be able to handle those filters.

“If you think of a filter as a tighter weave of all of these different fibers that are capturing the particles, the tighter that weave gets, the harder it is to get air through it, and older systems may not have the power or the fan strength to push air efficiently through a higher-rated MERV filter,” he said.

What if I have a window unit?

Older homes with leaky enclosures have a higher risk of increased particulate matter concentrations, according to ASHRAE. The group also noted that window and portable air-conditioning units should be avoided or used only in recirculation mode, which reuses the air already within a space.

If you don’t have an HVAC system, you can buy or build an air cleaner, Dr. DeCarlo said. The D.I.Y. versions are called Corsi-Rosenthal boxes and cost about $100 to make. “It’s a box fan, duct tape, and you need four or so of these higher-rated MERV filters to make kind of a cube,” he said.

In a post on LinkedIn, Rich Corsi, the dean of the College of Engineering at the University of California, Davis, who is a co-creator of the design, also noted that air quality may be improved by duct-taping rolls of activated carbon over supply and return vents in your home.

When should I use an air purifier?

Air purifiers should be used during increased air pollution, such as high-smoke events like the one currently blanketing parts of the United States and Canada. How many you need depends on the type of purifier.

Purifiers come in various sizes and strengths, Dr. Ghandehari said, noting that the appliances typically indicate the size of the space that they can clean.

“If you keep the door open to another room, then you have to realize that you’re not only serving the room where it’s located but the next-door room as well,” he said.

Dr. Ghandehari said that people can isolate themselves in the parts of their homes where they have air purifiers and that they should make sure there are no gaps through which smoky air can seep in.

How else can I keep my home safe?

Depending on your location and proximity to the source of the smoke, there’s a chance that you’ll need to wash your curtains, wipe down your blinds or even cover your furniture with a sheet that can be washed later, Dr. Ghandehari said. As examples, he pointed to Michigan and Wisconsin, where the A.Q.I. reached over 400 in some places. If smoke particulate embeds itself into the upholstery, he said, “every time you hit the upholstery, then it kind of raises in the air.”

Places farther away from the source of the current smoke, like New York, don’t necessarily need to be concerned with this because “some of it goes up in the upper atmosphere so the concentration comes down,” he said.