What to do about outside air quality?

Why should I care?

Outside air greatly affects our health. Air can rejuvenate us when fresh, but when polluted can cause a bunch of different issues. Polluted in this case means air which carries fumes, smoke, pollen, or fine/small dusts. Unfortunately, these pollutants trigger inflammation in your body, oxidative stress (damage to cells), and disrupt normal organ function. This in turn directly affects how you feel, making you tired, raising your stress or cortisol levels, lowering your focus, irritating your throat and lungs, or leading to other respiratory or even cognitive conditions.

It might surprise you, but in many parts of the world indoor air can be more polluted than outdoor air. Why? Because pollutants from cleaning products, furniture, cooking, gases humans produce or exhale, the dirt we bring indoors from outside on our shoes and clothing, even the materials our homes are made out of can build up over time, especially when spaces aren’t well ventilated.

That said, the outdoor air isn't always crystal clear. If you live in an area with heavy traffic, industrial production, or frequent wildfires, the air outside can sometimes be more of an issue than what’s indoors. In those situations, it’s worth being strategic by checking your local Air Quality Index (AQI) or pollution alerts before opening windows or heading outdoors. But more about that later. 

What is AQI?

Maybe the weather app on your phone says “Unhealthy air,” or you’ve seen a hazy skyline and wondered if it’s safe to crack a window or whether it’s the best time to take that walk. That’s where the Air Quality Index or “AQI” comes in. It measures things like fine particles (PM₂.₅), ozone, and nitrogen dioxide. You don’t need to remember all their names, just know they’re harmful pollutants from things like cars, chimney’s, industry, and you don’t want too much of them in your body. The AQI gives the outdoor air a score based on how much pollution air quality sensors measure is in the air at a given moment. 

"It’s the smallest particles are of special concern to scientists and can impact health the most” - George Thurston, Director of Exposure Assessment and Human Health Effects at NYU.

What do the AQI numbers mean?

Frustratingly, AQI isn’t measured the same way everywhere. In Europe the AQI or Common Air Quality Index (CAQI) goes from 0–100, while in the U.S. the EPA AQI ranges from 0–500. Both track similar pollutants and use the same color codes, but the numbers aren’t directly interchangeable.

Here are what the numbers mean in the European AQI (CAQI):

• 0–25 (Green): Good, the air is clean and safe.
• 26–50 (Yellow): Moderate, it’s fine for most people.
• 51–75 (Orange): Unhealthy for sensitive groups, be cautious if you have asthma or allergies.
• 76–100 (Red):  Unhealthy, limit outdoor activity and keep windows closed.
• 100+ (Dark Red): Very unhealthy, avoid outdoor exposure.

If you’ve seen these AQI values go over 100, you’re not crazy, it can happen. That’s because while the official AQI scale tops out at 100, some monitoring systems extend it to show extreme pollution levels beyond the “very high” category. In other words, anything above 100 is “off the charts” bad.

Here are what the numbers mean in the US AQI:

• 0–50 (Green): Good, the air is clean and safe.
• 51–100 (Yellow): Moderate, it’s fine for most people.
• 101–150 (Orange): Unhealthy for sensitive groups, be cautious if you have asthma or allergies.
• 151–200 (Red): Unhealthy, limit outdoor activity and keep windows closed.
  201–300 (Purple): Very unhealthy, avoid outdoor exposure.
• 300+ (Maroon): Hazardous, stay indoors and seal windows.

Always double-check which AQI scale your source is using, so you don’t accidentally worry yourself, or worse, think the air is fine when it’s not.

💡Tip from Birdie: Just like the weather, AQI changes throughout the day, especially in urban environments. It’s often lowest or best early in the morning or after rainfall, so those are the best times to ventilate or do some outdoor activities. If you have to go outside during a period of poor air quality, a mask can help reduce the amount of pollution you breathe in.

How does outdoor air or AQI affect your indoor climate?

As we’ve covered already, the air inside can often be more polluted than outside. But in some cases when the indoor air is polluted the outdoor air can be even worse. This could be due to a one off fire nearby, or an especially heavy traffic day. When you open a window on an especially polluted day, outdoor particles and gases flow in and mix with indoor air in the same way fresh air would. During these rare times it would actually be more beneficial to keep the windows closed and wait for the outdoor air quality to improve before ventilating again.

What can you do to breathe better indoors if you live in a polluted area? 

While outdoor air quality is largely out of your control, if the AQI is consistently high or during a period of poor air, here’s what you can do indoors:

• 🚪Seal cracks in windows and doorways to ensure outdoor air only enters on your terms.
• 💨Air purifiers with a HEPA filter help remove fine particles indoors. Make sure the air purifier is the appropriate size for the space you’re attempting to purify.
• 🪴Plants won’t replace ventilation or purify your air alone but they support humidity balance and add life to your space and clean it slightly if there’s a lot of them.
• 🕒Time when you ventilate during cleaner hours, because even in very polluted environments some times are better than others. Early morning or after rain are often the best times to open windows and doors.