PM2.5 is a size bracket, not a substance
PM2.5 is not a chemical, and it has no formula.
It is a size bracket ā any airborne particle 2.5 micrometres across or smaller, whatever it happens to be made of.
Soot from a wood burner, brake dust off the road, sea salt, the haze that lifts off a hot frying pan. All of it counts if it is small enough.
A human hair is roughly 60 micrometres wide. PM2.5 is about a thirtieth of that.
You cannot see an individual particle. What you see on a still winter morning is billions of them scattering the light.
The cut-off sits where the body stops filtering
Your nose and throat are a filter, and a reasonably good one.
The coarser bracket, PM10, covers particles up to 10 micrometres ā pollen fragments, crumbling masonry, the grit that makes a road feel dusty.
Most of that lands in the upper airways and is cleared within hours.
Below about 2.5 micrometres, that defence stops working.
Those particles reach the alveoli, the deep sacs where gas crosses into the blood, and the finest of them cross with it.
The threshold was drawn at 2.5 because that is roughly where inhaled dust stops being a nuisance and starts being a circulatory problem.
UK public health estimates put the burden of air pollution at the equivalent of 28,000 to 36,000 deaths a year, with fine particles carrying most of that weight.
The word "equivalent" is doing real work there. It is a population-level calculation, not a tally of death certificates.
The legal figure and the safe figure are not the same number
Three thresholds matter in England, and they sit a factor of five apart.
The limit retained from EU law is an annual average of 25 Āµg/mĀ³, and almost everywhere in Britain already sits comfortably below it.
Under the Environment Act 2021, England is bound to an annual average of 10 Āµg/mĀ³ by 2040, alongside a target to cut population exposure by more than a third.
The World Health Organization's 2021 guideline is 5 Āµg/mĀ³.
The figure England is legally working towards is still double what the World Health Organization considers a safe annual average.
Annual mean PM2.5 in micrograms per cubic metre: the limit retained from EU law, England's 2040 target, and the WHO guideline.
A typical urban background reading in a British city runs somewhere between 7 and 12 Āµg/mĀ³ averaged across a year.
Which is to say the old limit flatters us, and the WHO line does not.
Most of your dose is indoors, and some of it you make yourself
Outdoor air sets the floor. What happens in your kitchen sets the peak.
Frying, grilling and toasting under a grill can lift a kitchen into the hundreds of Āµg/mĀ³ for a few minutes at a time.
A candle does the same thing more slowly. So does an open fire, and so does a wood burner every time the door is opened.
Defra's national inventory puts domestic combustion ā overwhelmingly wood burning ā as the single largest source of PM2.5 in the UK, ahead of road transport.
That is the uncomfortable finding, because unlike traffic it is a source people choose.
Ventilation helps when the outdoor air is cleaner than the indoor air, which on a still, cold evening in a terraced street it often is not.
Checking before you throw the window open is a habit worth acquiring.
Check today's air quality forecastDefra UK-AIR, updated daily by region ā
Filtration is the other half of it. A HEPA-grade filter catches this bracket comfortably.
What decides the outcome is how much air the machine moves per hour, not the grade on the cartridge.
That figure is CADR, and we take it apart against the rest of the spec sheet in our Blueair Blue Pure 311i Max review.
An extractor hood that actually vents outdoors does more for a kitchen than any purifier, and costs nothing beyond the fan.
