The Law of Unexpected Consequences

Scientists have found electric, hybrid and other supposedly eco-friendly cars produce as much toxic particulate pollution as the “deadly diesels” they are meant to be replacing.

The tiny particles are produced by tyre and brake wear. This happens in all cars, including diesel and petrol, but eco-vehicles produce more because they are heavier, owing to the batteries and other parts needed to propel them. The extra emissions are roughly equal to the particulates saved by reduced engine use.

The added weight of eco-cars means that when they accelerate or slow down, the tyres and brakes wear faster, producing more particulates. The weight also whips up more particles from the road surface.

“We found that non-exhaust emissions, from brakes, tyres and the road, are far larger than exhaust emissions in all modern cars,” said Peter Achten, whose research is published in the journal Atmospheric Environment.

“These are more toxic than emissions from modern engines so they are likely to be key factors in the extra heart attacks, strokes and asthma attacks seen when air pollution levels surge.”

Achten, who runs a scientific consultancy in Holland, and his co-author Victor Timmers, of Edinburgh University, used technical data from the motor industry and government research agencies, including direct tests of brake, tyre and road wear rates, to show that the non-exhaust emissions produced by a vehicle are directly related to its weight. They also built a database of vehicle weights.

“We found that electric and eco-friendly cars typically weigh 24% more than conventional cars,” said Achten.

The findings fit with anecdotal complaints from electric and hybrid car owners that their tyres wear out faster than on conventional vehicles.

The impact of non-exhaust emissions has long been suspected but is hard to measure. Scientists at Hertfordshire University overcame this problem by installing particulate air pollution monitors in the southbound Hatfield tunnel on the A1(M), which carries up to 49,000 vehicles a day.

They found each vehicle produced 34-39 micrograms of particles per kilometre but only a third came from the engine.

The rest comprised mainly tiny pieces of bitumen whipped up from the road, rubber from tyres and brake dust. In towns — where cars brake and accelerate more often — this proportion may reach 90%.

Such findings are a problem for policy-makers whose anti-pollution efforts have been focused on regulating engines. Professor Ranjeet Sokhi, of Hertfordshire University, who led the study, said: “This highlights the significance of non-exhaust emissions and a need for legislation.”

The Society of Motor Manufacturers and Traders said the industry was working to make eco-friendly cars lighter. “Such vehicles have zero or ultra-low tailpipe emissions and have energy recovery systems, which limit the need for active braking, reducing brake and tyre friction that may contribute to particulate emissions.”

Toyota, a market leader in hybrid, plug-in hybrid and fuel-cell cars, said the firm had no data on particulate emissions from brakes and tyres but added: “An advantage of hybrid cars over diesel is that nitrogen dioxide and hydrocarbon emissions are incomparably better.”

Frank Kelly, professor of environmental health at King’s College London, said one hope lay in changing the composition of tyres and road surfaces. “Non-exhaust PM [particulate matter] emissions are greater than exhaust and we do not have regulations to deal with these emissions.”

A Department for Transport spokesman said eco-vehicles still had huge benefits in cutting CO2 emissions.