A team of French scientists has been investigating air pollution from bonfires. They used a specifically constructed fire chamber: a big room that could easily accommodate one or two whole houses, added instrumentation in the extract ducts, spread a bed of sand on the floor and set about burning leaves and hedge trimmings.

Bonfires are a frequent source of complaints to UK local councils, and in some places these complaints quadrupled during the 2020 lockdown. But little is known about the air pollution they cause. This means they are often assumed to produce pollution that is similar to home fires and wood stoves.

Any gardener (and their neighbours) will know the smell of smoke from burning green waste. Unsurprisingly, for each kilogram burned, garden waste on bonfires produced up to 30 times more particle pollution (smoke) than burning logs in a stove, but smoke from the wood stove contained up to 12 times more cancer-causing polyaromatic hydrocarbons. The pollution from bonfires more closely resembled wildfire smoke, which is being increasingly linked to health problems.

Autumn is coming and so is the annual garden-tidy before winter. The simple message is: do not burn your garden waste; compost it instead or shred it to make a mulch.