Quick answer: An AQI of 171 sits in the "Unhealthy" band (151–200), driven almost entirely by PM2.5 — the fine particles in wildfire smoke. At that level, everyone can feel effects and sensitive groups are at real risk: eye and throat irritation, coughing, headaches, shortness of breath, and aggravated asthma or heart conditions. The particles are small enough to reach deep into your lungs and cross into your bloodstream. You can't change the outdoor number, but you can dramatically lower what you actually breathe by cleaning your indoor air.
With Canadian wildfire smoke pushing New York and the Northeast into the Unhealthy range this week, "AQI 171" stopped being an abstraction. Here's what that number is measuring and what it does.
What the AQI is actually counting
During a smoke event, the AQI is really a PM2.5 reading — particulate matter 2.5 microns across or smaller. For scale, that's about 30 times thinner than a human hair. Wildfire smoke is overwhelmingly made of it. Learn the particle science in our air-quality guides, but the one fact that matters: PM2.5 is small enough to bypass your body's natural defenses.
What 171 does to your body
Coarser dust gets caught in your nose and throat. PM2.5 doesn't. It travels to the deepest part of the lungs — the alveoli, where oxygen crosses into blood — and the finest particles pass into the bloodstream itself. That's why the health effects reach well beyond the lungs:
- Immediate: burning eyes, scratchy throat, coughing, wheezing, headache, fatigue, shortness of breath.
- Respiratory: asthma attacks, bronchitis flare-ups, reduced lung function.
- Cardiovascular: inflammation that can raise the risk of heart attack and stroke, which is why cardiac ERs see more visits during smoke events.
At an AQI over 150, the health agencies stop distinguishing between "sensitive" and "everyone" — the general population is now at risk, and sensitive groups more so.
Who's most at risk
- People with asthma, COPD, or other lung disease
- People with heart disease
- Older adults and young children (whose lungs are still developing)
- Pregnant people
- Anyone doing heavy outdoor exertion, which pulls more particles deeper
The one lever you control: indoor air
You can't lower the AQI outside. But you spend most of a smoke day indoors, and indoor air is fixable. Three moves, in order of impact:
- Stop the inflow. Close windows and doors and set your AC to recirculate, not fresh-air intake.
- Filter what's already inside. Set your thermostat fan to "On" so it runs continuously, with a MERV 13 filter. MERV 13 removes at least 85% of PM2.5 on each pass — the whole reason it's the smoke-and-allergy standard. Higher MERV 14–16 captures even more if your system supports it.
- Build a clean-air room. Put a HEPA purifier or a Corsi-Rosenthal box in the bedroom so at least one space stays low all day and night.
Add a carbon filter if the smoke smell is getting in — carbon adsorbs the odor gases that particle filters can't. And watch airflow: during heavy smoke a filter loads fast, so check it more often, especially on older systems.
Frequently asked questions
Is AQI 171 safe to go outside in? It's Unhealthy for everyone. Brief essential trips are fine with an N95; avoid prolonged or strenuous outdoor activity, and keep kids and at-risk people indoors.
Can I feel PM2.5 or does it just smell? Both. Beyond the smell, common signs are irritated eyes, a scratchy throat, a dull headache, and feeling winded doing normal things. Those are your cue to get indoors and filter the air.
Does a MERV 13 furnace filter really lower PM2.5 indoors? Yes — run the fan continuously and it removes the majority of fine particles from every room the system serves. It's the most cost-effective whole-home step during a smoke event.
What AQI is considered safe? 0–50 (Good) is ideal; 51–100 (Moderate) is generally acceptable. Sensitive groups should start taking care above 100, and everyone above 150.
This is general health information, not medical advice — if you have a heart or lung condition and your symptoms are worsening, contact your doctor or local health guidance. Ironside makes American-made air filters in every standard size, MERV 6 to 16, shipped flat-rate nationwide, the same on 1, 2 or 3 packs. Find your size and breathe better.