Air quality advice tends to arrive as a number and an instruction to stay indoors. Both are less useful than they sound, because the number hides what is actually in the air and indoors is often not much better.
What the index is really telling you
An air quality index converts concentrations of several pollutants into one figure, and reports whichever is worst. Two cities showing the same value can therefore have very different air: one driven by fine particulates, another by ozone. The pollutant matters, because the protective measures differ.
For most urban smog the driver is PM2.5, particles under 2.5 micrometres across. They are small enough to pass deep into the lungs and into the bloodstream, which is why they are associated with cardiovascular as well as respiratory effects. Ozone, by contrast, is a gas and is not filtered by any mask.
Masks: which ones do anything
- Respirators rated N95, KN95 or FFP2. These filter fine particulates effectively, but only with a proper seal against the face. Gaps at the sides defeat them.
- Surgical masks. Designed to catch droplets outward, not to seal. Limited benefit against PM2.5.
- Cloth masks. Effectively no protection against particles this small.
- Any mask against ozone or nitrogen dioxide. None work. These are gases, and particulate filters do not remove them.
Indoors is not automatically safe
Fine particulates pass through gaps around windows and doors, so indoor levels track outdoor levels with a delay unless something is actively removing them. A HEPA air purifier sized for the room genuinely reduces indoor PM2.5, and a cheaper alternative is a box fan with a filter attached, which works on the same principle.
Indoor sources matter too, and are easy to overlook during an outdoor episode: cooking without extraction, burning candles or incense, and any indoor smoking all raise PM2.5 substantially in a closed room.
Timing and exposure
Exposure is concentration multiplied by time multiplied by how hard you are breathing. Exercise increases the volume of air you inhale several times over, so a run during a bad episode delivers far more pollutant than a walk. Pollution also varies through the day, commonly worse in cold, still early mornings when a temperature inversion traps air near the ground.
The practical response is to shift activity rather than abandon it: exercise indoors on bad days, avoid busy roads where roadside concentrations are far higher than the city average, and treat the hours after rain or wind as the better window.
Common questions
Is a lower index number always safer? Broadly yes, but the health evidence suggests no threshold below which fine particulates are harmless. Lower is better rather than safe.
Do house plants clean the air? Not at any meaningful rate. The studies often cited used sealed chambers, and the effect does not scale to a ventilated room.
Are air purifiers worth it? For particulates, yes, provided the unit is rated for the room size and the filter is replaced. They do nothing for gases unless they include activated carbon.
Who is most affected? Children, older adults, pregnant women, and anyone with asthma or heart disease. Effects appear at lower concentrations in these groups.
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