If the sky is feeling just a tad apocalyptic where you live, you might want to familiarize yourself with the Air Quality Index.

While California, the Pacific Northwest, and Mountain West have been feeling the effects of intense, climate change–fueled wildfires for years, and have become depressingly accustomed to navigating the days when the skies are orange and smoky, New York City and other eastern seaboard metropolises are now getting to experience this eerie environment for themselves. Smoke and ash from unusually large and intense wildfires burning in Canada are traveling South and East, suffusing places like Manhattan and Brooklyn with the oppressive aromas of a city-wide barbecue, according to The New York Times.

If this is your first time experiencing the infernal whims of the climate, know that you do have some tools at your disposal. Meet the Air Quality Index (AQI), your guide to whether you can leave your house or not. Okay, it’s more than that. This governmental measure of how polluted the air is on a scale of zero to 500 will tell you who is at risk for irritating their lungs and breathing passages. If you know how to interpret the scores, the AQI will give you an idea of whether to do activities inside or outside, whether you should be running those air purifiers, and if you should bust out your N95 mask.

What is the Air Quality Index?

The government takes air samples at places all over the country, measuring the density of five pollutants, including: ground-level ozone, particle pollution (also known as particulate matter, including PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. It then breaks down those concentrations into an easy-to-understand rating from an Air Quality Index of zero to 500, with zero signaling pure, clean air, and 500 reflecting a totally unlivable environment. Anything over 300 falls into the “hide yo kids hide yo wife” category. But things can still get dicey between 100 and 300.

For context, the sepia-colored streets of New York City are currently registering AQIs of 160 in South Brooklyn and 370 on the Upper East Side.

AQI ratings are especially important for people with certain health conditions like asthma to consider. Ratings of 50 to 150 are unhealthy for sensitive groups, while that level of pollution is not likely to affect the general population. Ratings of 151 to 200 could affect “some members of the general public,” and anything over 200 puts everyone at risk.

What’s more, you should be checking those numbers—it’s really not enough to just look at the sky and breathe in to assess the situation.

“Just because the sky looks fairly clear, the air quality can still be fairly poor,” Russell Buhr, MD, PhD, a pulmonologist at UCLA Health, previously told Well+Good.

You should also get into the habit of checking the AQI because you could be feeling the environmental effects of a wildfire that’s not even on your radar.

“Wildfires do not only affect people in the immediate fire area,” Kenneth Mendez, CEO and president of the Asthma and Allergy Foundation of America, says. “Smoke can blow many miles away and impact people hundreds and even thousands of miles away.” Hence, the effects of a Canadian fire showing up in NYC.

And that’s important to pay attention to, even if you don’t think of yourself as someone who needs to be extra careful. “Smoke and ash contain harmful particles that can irritate even healthy lungs,” says Mendez.

How a high AQI day could affect your health

Chronic exposure to pollution can lead to a host of health problems, including “stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma,” according to the World Health Organization.

While a bad AQI day here and there may not put you at risk for those conditions, it could have more immediate effects.

“The nose’s normal function is to filter impurities from the air and moisten and warm up the air so that clean, moist, warm air gets to the lungs,” Sujana Chandrasekhar, MD, an otolaryngology partner at ENT and Allergy Associates, says. “When the air is so impure, the functionality of the nose as the first-line filter gets greatly reduced. If you breathe in through your mouth because your nose is irritated, the filtering and warming/moistening activities do not work.”

And as you breathe smoky air in, it irritates the lining of your nose, mouth, throat, windpipe, and lungs. This can lead to breathing issues like coughing and wheezing.

“Anything that irritates as it passes your larynx, or voice box, to get to your lungs will cause you to cough or clear the throat, which can itself cause further irritation in that area—causing hoarseness or breathiness, including an inability to complete a sentence in one breath.”

Additionally, burning or hot particles can singe the hairs on the inside of the nose or burn your lips, nasal lining, or oral lining. “When the nasal lining gets so dry, people are prone to nosebleeds. The more frequently you have nosebleeds, the longer it takes for recovery,” she says. Additionally, poor air quality can make your allergies and environmental sensitivities worse. “Bad air quality outside can make your dust or dog allergy inside act up.”

How to protect yourself in poor air quality

First things first, make sure you’re checking that AQI. You can do that by Googling “AQI near me,” which should take you to a map where you can zoom in on your location. You can also open Google Maps, click the layers button, and turn on the air quality and/or wildfires filters. Google will break the ratings down into “good,” “moderate,” and “poor.”

Now, here’s what to do when the AQI is poor.

Avoid exercising outside

When you work out, you’re breathing in five to six times more air than normal breathing, according to Dr. Buhr. This is the time to minimize the amount of polluted air you’re breathing in by sticking to indoor workouts.

Wear a mask outdoors

If you do go outside, it’s time to reacquaint yourself with the N95 mask (a cloth or lower-rated mask won’t do the trick).

“If air pollution is at hazardous levels, an N95-rated mask would provide protection for your airways,” Dr. Chandrasekhar says. If you’re using an old mask, make sure you’ve worn it fewer than five times before.

Maintain (and purify) indoor air

Keep your windows and doors closed to the polluted air outside. Since seepage is likely, it’s also a good time to run that air purifier.

“You can get a small HEPA-filter type air purifier that will work really well in a room,” she says. “You don’t have to do a big HVAC job to get the benefit.”

Another idea? “Cool-mist humidifiers are also nice,” Dr. Chandrasekhar says. They keep the air moist, which makes small particles in the air get heavy and fall down so you don’t breathe them in.

Stay hydrated

When the air quality is poor your skin can get dry and lead to mouth breathing, which leads us to lose more fluids.

“When our mucous membranes (lining of nose/mouth/throat/air passages) are dry, they become inefficient,” says Dr. Chandrasekhar. “Remember how bad things taste when your mouth is really dry? It’s because your taste buds are not up to par in that condition. Hydrating up is very restorative.”

During times of poor air quality, Dr. Chandrasekhar suggests using a saline mist, spray, or gel in your nose frequently.

“I’m a big fan of saline gel,” she says. “It’s easy to apply and very soothing—and lasts for a long time.”

Intentionally breathe through your nose

Remember what we said about your nose being the first line of defense against pollution? It’s time to put that barrier to work. You can get in the habit of employing those nostrils by doing breathing exercises.

“There’s a type of yogic breathing called pranayama with controlled breaths in and out,” Dr. Chandrasekhar says. “This is a healthy and restful way to keep sure that your air passages are getting the proper ‘exercise’ and are able to withstand some degree of atmospheric change. It is also a great way to quell anxieties, which are rampant especially in bad times like these.”

A simple pranayama exercise you can try is breath extension. Start by inhaling for four to five counts through the nose. Hold for four counts. Then exhale deeply through the mouth while making a sound. Repeat.