Tire emissions up to 1,000 times worse than tailpipe emissions, study finds

Just when you thought it was environmentally safe to test drive a zero-emissions truck, a new study suggests tires in some cases could produce emissions 1,000 times worse than tailpipe emissions.

Blame the wear and tear from the rubber meeting the road, say the researchers at Emissions Analytics, a U.K.-based global testing and data specialist for the scientific measurement of real-world emissions.

The past decade has seen a raft of state policies aiming to tighten tailpipe pollution regulations. California is front and center, with its Truck and Bus rule that aims to upgrade the heavy-duty trucking fleet, as well as a new clean truck proposal that would require manufacturers to sell a certain number of zero-emissions vehicles.

More recently: The Washington State Senate passed a bill this week that will make it the 12th state to adopt California’s zero-emissions policy. The policy requires that at least 5% of all vehicles sold, including medium-duty vehicles, be electric. By 2025, EV sales must make up 8%.

Despite the attention paid to cleaning up dirty cars and trucks, the definition of zero emissions has thus far focused on tailpipe exhaust. For the most part, regulators have yet to take aim at non-exhaust sources.

Apparently they should. 

Using a “popular family hatchback,” the Emissions Analytics team found that the car’s brand new, correctly inflated tires emitted 5.8 grams of particulate matter per kilometer. Exhaust emission from the same vehicle clocked in at 4.5 milligrams per kilometer.

Tires degrade with use, creating particles that enter the air. Brake pad and road surface wear are also major contributors to particulate matter, the researchers said.

The study comes with a few qualifications. The make of the vehicle tested was not immediately clear. And although the researchers could not be reached by press time, a comment from Emissions Analytics appended to a write-up in Green Car Reports explains researchers were trying to quantify the worst-case scenario: “the vehicle with a heavy payload, driven aggressively on cheap tyres. This has enabled us to get an idea of how big a problem tyre wear emissions might be.”

Emissions Analytics is undertaking more testing under the opposite scenario: low payload, normal driving and expensive tires, the comment stated, and expects the difference in tire wear emissions to be about two orders of magnitude. “Even in that case, tyre emissions would be at least x10 the particle mass at the tailpipe,” the comment states.

Experts agree tire and brake-related pollution is only going to get worse. According to a study presented at the International Emissions Inventory Conference in 2015, non-exhaust sources are going to make up an increasing proportion of the particulate matter in the air.

That’s due to the fact that tailpipe emissions have declined dramatically, even as sales of heavier, battery-powered EVs — which put more pressure on tires — are increasing, Emissions Analytics senior researcher Richard Lofthouse said in a press release.

Non-exhaust emissions are “a very serious problem,” he said.

Acknowledging the seriousness of the issue, California Air Resources Board, the state’s air quality regulator, has contracted with several public and private research groups to study the impact of brake and tire wear on emissions.

In one of these projects, the Eastern Research Group and LINK Engineering are varying brake materials, vehicle loads and driving behavior to explore factors affecting particulate matter emitted by these sources. This two-year project launched in late 2018.

“With increasingly stringent standards for exhaust emissions, the non-exhaust fraction has become increasingly important,” the CARB website states.

Nick Molden, CEO of Emissions Analytics, echoed that view.

“The challenge to the industry and regulators is an almost complete black hole of consumer information, undone by frankly out of date regulations still preoccupied with exhaust emissions,” Molden said in the release. “What is without doubt on the horizon is much-needed regulation to combat this problem.”