The UK government's Renewable Heat Incentive (RHI) aims to transform domestic and non-domestic heating with affordable biomass-only boilers and pellet stoves. This could have unwanted consequences for air quality and climate change unless the RHI rapidly reduces its emissions limits (see also J. Schmale et al. Nature 515, 335–337; 2014).

The RHI stipulates an air-quality limit for biofuel burning of 30 grams of particulate matter per gigajoule of net heat output. This means that a 10-kilowatt unit operating for 2.8 hours is allowed to emit as much as 3 g of particulates of up to 10 micrometres in size (PM10) in a total of 27 cubic metres of boiler exhaust fumes (111 milligrams per cubic metre). Under current UK air-quality regulations, the permitted ambient PM10 level is about 2,000 times lower than this; the European PM10 limit is 5,000 times lower.

The RHI impact assessment suggests supporting 750,000 systems by 2020. Yet, if just 2.5% of the UK homes now heated by natural gas (500,000 boilers) were to switch to RHI-compliant 10-kW biomass boilers (enough to power five double radiators) and use them for 2.8 hours, this would generate the same mass of particulates as would be emitted in one day by the entire UK light-duty diesel fleet of 8.8 million vehicles were they to operate under the latest Euro 5 emissions regulations (see

Furthermore, much of the particulate matter emitted by biomass boilers is sooty black carbon — a climate-warming agent owing to its absorption of outgoing longwave radiation. So the intended positive effect of the RHI on climate will be compromised.