Vancouver tops North American cities in toxic benzene: study

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Long-term exposure to benzene has been found to cause blood cancers such as leukemia.

Gas heaters in several homes in Vancouver, B.C., were found to produce the most toxic benzene emissions of 17 North American cities analyzed in a new study.

The study, published Tuesday in the journal Environmental Research Letters by researchers from Stanford University and the PSE Healthy Energy research institute, examined 481 unburned natural gas samples collected in 2021 and 2022.

After testing for hydrocarbons and dozens of dangerous air pollutants, researchers found that 97 percent of the samples contained benzene – a toxic substance that has been found to cause blood cancers such as leukemia with chronic exposure.

But the cities varied greatly in the amount of benzene in their gas systems. Samples from Vancouver were found to have average benzene concentrations nearly double those of Los Angeles, Calgary and Denver, the next three cities with high benzene levels, and 50 times higher than those of Boston, the city with the lowest concentrations.

Drew Michanowicz, a senior scientist at PSE Healthy Energy in Boston and the study’s supervising author, said that because benzene is a known carcinogen, there are actually no safe exposure levels.

“It is the most toxic compound we find in natural gas,” says Michanowicz. “So yeah, I would say it was quite surprising to find the levels that we found in Vancouver.”

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Distribution of benzene in natural gas by city and ranked by average concentration in the city (black horizontal bar). Each point represents a natural gas sample. Rowland et al., 2024

Michanowicz said the cities in the study were chosen based on population size, geography and proximity to the oil and gas industry. Previous studies had shown that Californian cities had particularly high levels of benzene, and Michanowicz said the team was curious whether these numbers were reflected on Vancouver’s coast.

Dave Risk, an emissions measurement scientist and head of the Flux Lab at St. Francis Xavier University in Nova Scotia, said the large differences in benzene concentrations likely come from different regions getting their gas supplies from different production reservoirs.

Geological processes that have occurred over millions of years mean the composition of gas in northeastern B.C. and Alberta will be different than that on the U.S. East Coast, said Risk, who did not participate in the study.

“Natural gas is a bit like soup,” he said. “Depending on where we come from, we will naturally have different benzene concentrations in the natural gas.”

A stinking risk

In each sample, the researchers also measured concentrations of organic sulfur odorants, the rotten egg odor that gas companies add so people can recognize a gas leak.

Thirteen houses in the study were found to have gas leaks that the residents did not smoke. That’s a concern for people who have trouble smelling — either due to age or an olfactory disorder caused by an illness like COVID-19, Michanowicz said.

“It’s the only safety system that exists,” Michanowicz said. “We really just rely on our individual noses.”

“But in some samples we found no detectable odorant at all.”

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Researchers take gas samples from a stove. Alessandro Citterio

The study estimates that U.S. and Canadian emissions inventories fail to account for 29,000 pounds of benzene emissions annually from natural gas leaks.

Long-term, chronic exposure to all that benzene could lead to more cancer cases, and when released into the atmosphere, much of the methane gas acts as a greenhouse gas more than 80 times more potent than carbon dioxide over 20 years.

Then there is the very short-term risk: a gas explosion that engulfs someone’s home.

Risk said the regulatory goals set for odor gas are quite broad, so it’s no surprise that their levels varied from city to city. What did surprise him was that they failed to match the concentration of benzene in the gas.

“It’s a concern,” Risk says. “The fact that the fragrance doesn’t fully protect us is, I would say, information and something that we need to pay attention to and that regulators may want to look at.”