Air Pollution: What PM2.5 Does and What Actually Protects You
On bad days in many cities, the air quality index is a number everyone quotes and few people can interpret. Here is what the measurements mean and which protective measures are supported by evidence rather than marketing.
What is in polluted air
PM2.5. Particles smaller than 2.5 micrometres. These are the ones that matter most for health, because they are small enough to pass deep into the lungs and cross into the bloodstream. Sources include vehicle exhaust, industrial emissions, biomass and crop burning, construction dust and cooking fuel.
PM10. Larger particles, mostly trapped in the upper airway, causing irritation.
Nitrogen dioxide, largely from traffic, which irritates airways and worsens asthma.
Ozone, formed by sunlight acting on pollutants, peaking in the afternoon, a powerful airway irritant.
Sulphur dioxide and carbon monoxide, from combustion.
Why PM2.5 is the headline
Because of size. Particles this small evade the nose and upper airway defences, deposit in the small airways and alveoli, and provoke inflammation. A fraction crosses into the circulation.
The health effects therefore extend well beyond the lungs. Air pollution is associated with heart attacks, stroke, hypertension, worsened diabetes, reduced lung growth in children, low birth weight, and increasing evidence links it to cognitive decline.
The World Health Organization’s guideline for annual average PM2.5 is 5 µg/m³, with a 24-hour guideline of 15 µg/m³. Many cities exceed these by large multiples. Importantly, no threshold has been identified below which there is no effect.
Reading the AQI
AQI is a converted scale, and different countries use different conversions, so the same raw concentration can produce different AQI numbers. Broadly:
- 0 to 50, good. Normal activity
- 51 to 100, moderate. Unusually sensitive people may consider reducing prolonged outdoor exertion
- 101 to 150, unhealthy for sensitive groups. Children, older adults, pregnant women, and people with heart or lung disease should limit prolonged outdoor exertion
- 151 to 200, unhealthy. Everyone should reduce prolonged outdoor exertion
- 201 to 300, very unhealthy. Avoid outdoor exertion; sensitive groups stay indoors
- Above 300, hazardous. Everyone should remain indoors with filtration
Check the PM2.5 value directly if available; it is more comparable across sources than the AQI band.
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Masks: what works
N95 or FFP2 respirators filter at least 94 to 95 per cent of fine particles and are the only widely available option that meaningfully reduces PM2.5 exposure. Fit is everything: a gap at the nose or cheeks lets unfiltered air bypass the filter entirely. Facial hair prevents a seal.
Surgical masks are designed to block droplets, not fine particles, and fit loosely. They offer limited protection against PM2.5.
Cloth masks offer very little.
Valved respirators protect the wearer but vent unfiltered exhaled air, which matters for infection control but not for pollution.
N95s are uncomfortable for long periods and are best reserved for necessary outdoor exposure on bad days, not worn continuously.
Air purifiers: worth it, with conditions
A HEPA purifier genuinely reduces indoor PM2.5, and studies have shown measurable improvements in blood pressure and respiratory symptoms with their use.
What matters when choosing:
- True HEPA filtration, which captures at least 99.95 per cent of fine particles. “HEPA-type” or “HEPA-like” is marketing language for a lesser filter
- Clean air delivery rate matched to room size. An undersized unit in a large room achieves little
- Activated carbon if gases and odours are a concern; HEPA does not remove them
- Running it with windows and doors closed, which is how it was tested
- Filter replacement on schedule, since a saturated filter stops working
What to avoid: ozone-generating purifiers and some ionisers, because ozone is itself a lung irritant and the devices are not effective at particle removal.
A purifier in the bedroom, running overnight, gives the best return for a single unit, because that is where most people spend the longest continuous period.
Indoor air is not automatically clean
Indoor PM2.5 tracks outdoor levels closely unless filtration is used, and indoor sources add to it: cooking, particularly frying and use of biomass or kerosene, incense and candles, smoking, mosquito coils, and unvented heaters.
Practical measures: use an extractor hood or open a window while cooking, switch to cleaner cooking fuel where possible, avoid burning incense and coils in closed rooms, and never smoke indoors.
Practical behaviour on bad days
- Check levels before planning outdoor exercise, and move it indoors on high days. Exercise increases ventilation several-fold, so you inhale proportionally more
- Exercise in the early morning in many cities, when traffic-related pollutants are lower, though this varies locally
- Keep windows closed during peak pollution and ventilate when levels drop
- Avoid walking or cycling along heavily trafficked roads; moving a few streets away reduces exposure substantially
- Keep car windows up with recirculation on in heavy traffic
- Ensure asthma and COPD treatment is optimised before the polluted season, not during it
Who should be most careful
Children, whose lungs are still developing and who breathe more air per kilogram of body weight; older adults; pregnant women; and anyone with asthma, COPD, heart disease or diabetes.
The short version
PM2.5 is the number to watch, and its effects reach well beyond the lungs. On bad days, a properly fitted N95 for necessary outdoor time and a correctly sized true-HEPA purifier running in a closed bedroom are the two measures with real evidence. Most other products sold for air quality are not.