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New research finds that even people with an average sense of smell may be living with a natural gas leak and not know it. The study, published in Environmental Research Lettersfound that small gas leaks can affect indoor air quality by introducing a number of dangerous air pollutants, including the carcinogenic benzene, which researchers found in 97 percent of North American natural gas samples.

“Although these smaller leaks are not large enough to cause gas explosions, foul-smelling leaks are common,” said lead author and PSE healthy energy scientist Sebastian Rowland. “The fact that they are so small makes them difficult to identify and fix, which can lead to a persistent indoor source of benzene and methane.”

The study is the first to assess whether leaking gas contains enough odorants to protect against increased levels of benzene exposure, and provides the most comprehensive data to date on the composition of natural gas in residential spaces.

Researchers from PSE Healthy Energy and Stanford University collected and analyzed 587 natural gas samples from 481 residences in 17 North American cities. The samples were tested for methane, hazardous air pollutants, and sulfur-based flavorings, and the researchers modeled the amount of gas that could leak undetected by a resident with an average sense of smell. Their findings confirm that benzene and other dangerous air pollutants are present in nearly all natural gas delivered to homes, buildings and businesses across North America.

“Our nose is the first and only line of defense against a gas leak at the household level,” said PSE Senior Scientist Drew Mihanovich. “Given the variation in odor levels and the huge disparity in smell ability among the general public, our findings really call into question the sole reliance on smell to protect people from gas leaks.”

On average, gas delivered to homes in Vancouver, Los Angeles, Calgary and Denver had twice the benzene levels of other cities, with Vancouver’s benzene levels nearly fifty times higher than the city with the most -low concentration, Boston. Odorant levels in gas from Houston were roughly five times higher than those in Toronto, while neighboring cities New York and Washington, D.C., appeared to use completely different odorant products, indicating a lack of standardization.

Average measured levels of natural gas odorants should alert most occupants to a gas leak, which could result in high benzene exposures; however, each household faces a different situation due to differences in personal odor sensitivities, ventilation rates, gas composition, and barriers to remediating leaks after detection.

The researchers suggest that regulators and consumers would benefit from greater transparency of natural gas composition, such as open access to natural gas composition data and regular sampling. Additionally, improving leak detection through stricter odorization standards or increased use of leak detection devices, or reducing gas use altogether, can improve indoor air quality and public health.