// the science, particulate matter
What is PM2.5?
PM2.5 is fine particulate matter, airborne specks 2.5 micrometres across or smaller, roughly 1/30th the width of a human hair. They're too small to see, taste, or feel. They're also small enough to slip past your airways' defenses, lodge deep in your lungs, and cross into your bloodstream.
of people breathe air above the WHO annual guideline of 5 µg/m³.
deaths a year are linked to air pollution, the 2nd-leading risk factor for death worldwide.
the WHO annual PM2.5 guideline. Most of the world sits at multiples above it.
How small is “2.5”?
The 2.5 is micrometres, millionths of a metre. For scale:
- Human hair~50–70 µm
- Pollen / dust (PM10)≤ 10 µm
- PM2.5≤ 2.5 µm
// bars relative to a human hair, PM2.5 is genuinely tiny
Where it comes from
Anything that burns. And a lot of it happens indoors, where most people spend ~90% of their time.
Cooking
Frying, searing, and gas stoves can push a kitchen past 200 µg/m³ in minutes.
Traffic & combustion
Diesel, brakes, and tyres, the reason that one tunnel on your commute spikes.
Wildfire smoke
Travels thousands of kilometres; turns 'good' regions hazardous for days.
Candles & incense
Cosy, and a steady indoor source of fine particles.
What it does to your body
Because PM2.5 crosses from the lungs into the blood, the damage isn't only respiratory. Around 90% of air-pollution deaths are non-communicable diseases:
- · Heart disease & stroke
- · Lung cancer
- · COPD & asthma
- · Type 2 diabetes
- · Low birth weight & preterm birth
- · Links to dementia & cognitive decline
Crucially, there is no known safe level. Risk keeps rising with exposure and persists even below the WHO guideline, which is why the guideline is a target, not a finish line.
The colour scale
Annual-mean PM2.5 in µg/m³, by air-quality category. The WHO guideline sits at just 5 µg/m³, at the very bottom of the first band.
- 0–12Good
- 12.1–35.4Moderate
- 35.5–55.4Unhealthy for Sensitive Groups
- 55.5–150.4Unhealthy
- 150.5–250.4Very Unhealthy
- >250.4Hazardous
The studies that proved it
Harvard Six Cities Study, NEJM, 1993
Followed ~8,000 adults across six US cities for 15 years. People in the most-polluted city were ~26% more likely to die than those in the cleanest, the first hard link between fine particles and mortality.
Pope et al., JAMA, 2002
Tracked ~500,000 adults. Each 10 µg/m³ of long-term PM2.5 was associated with roughly 4% higher all-cause, 6% higher cardiopulmonary, and 8% higher lung-cancer mortality.
Pope et al., NEJM, 2009
As US cities cleaned up their air, a 10 µg/m³ drop in PM2.5 tracked with about a 0.6-year gain in life expectancy. Cleaner air measurably buys time.
Sources
- WHO, Global Air Quality Guidelines (2021): annual PM2.5 5 µg/m³, 24-hour 15 µg/m³
- WHO, Ambient (outdoor) air pollution fact sheet (99% of people; ~4.2M deaths)
- Health Effects Institute, State of Global Air 2024 (8.1M deaths; 2nd-leading risk factor)
- Dockery et al., Harvard Six Cities Study, New England Journal of Medicine (1993)
- Pope et al., Lung cancer, cardiopulmonary mortality & long-term PM2.5, JAMA (2002)
- Pope et al., Fine-particulate air pollution & life expectancy, NEJM (2009)
You can't manage what you can't see.
City maps show yesterday's outdoor average. Your real exposure is personal, the kitchen, the commute, the office, and it changes by the minute. Partycle is a portable sensor that tracks the PM2.5 you're actually breathing, indoors and out.