The average American spends almost one hour each day commuting to work, a number that adds up quickly.
Now, research has found that Californian car commuters can be exposed to above-acceptable levels of unhealthy chemicals during their daily work trips.
Estimating commuter times from census data and using measurements of chemicals detected in previous studies, the new study found commutes of more than 20 minutes put people at risk of unacceptably high levels of two carcinogens used in car manufacturing.
At first glance, it might seem like worrying news for people who spend a lot of time behind the wheel on daily commutes. But there are a lot of factors at play, so let's unpack it a little.
The distinct smell of a new car gives you a whiff of what's happening here. Materials used to fit out cars, from hard and soft plastics to adhesives, textiles and foam, contain some chemicals that can slowly seep into the air (the technical term is 'off-gas') or catch a ride on dust.
"These chemicals are very volatile, moving easily from plastics and textiles to the air that you breathe," said environmental toxicologist David Volz from the University of California Riverside, who co-authored the study.
Volatile compounds can build up in small spaces, such as inside a car (unless you open a window for fresh air).
While much research to date has focused on outdoor air pollution and its impact on health, and indoor environments, such as workplaces or homes where people spend most of their days, this study suggests chemicals building up inside vehicles could also be a concern - to some drivers.
The study aimed to estimate when a person's exposure to known carcinogens likely tipped over safe thresholds based on the time commuters spent inside their vehicles, and on the levels of five chemicals detected inside cars in previous studies.
The researchers predicted that commuters' daily exposure to two out of five chemicals studied - benzene and formaldehyde - would probably exceed levels considered safe or allowable by Californian health authorities after 20 minutes. The likelihood continued to rise the longer a person's commute.
The two chemicals of concern are not huge unknowns, but certainly warrant attention. Benzene is found in rubber and dyes, and formaldehyde is used in carpets and paints, and both are included on California's Proposition 65 lengthy carcinogen list. What's new here is looking at the risk these chemicals may pose to drivers specifically.
"Our study raises concerns about the potential risk associated with inhalation of benzene and formaldehyde for people who spend a significant amount of time in their vehicles, an issue that is especially pertinent to traffic-congested areas where people have longer commutes," the study authors wrote.
Perhaps unsurprisingly, a greater fraction of people had an elevated chance (above 1 in 10) of exceeding cancer risk thresholds for benzene and formaldehyde on their daily commute in areas around San Francisco and the notoriously traffic-congested Los Angeles.
"Of course, there is a range of exposure that depends on how long you're in the car, and how much of the compounds your car is emitting," which can depend on the vehicle's age and surrounding temperatures, said graduate student and study lead author Aalekhya Reddam, also from the University of California Riverside.
In other words, just because something is listed as a carcinogen doesn't mean it's guaranteed to cause health problems – it depends on the dose that people are exposed to, and how often.
Even known carcinogens are not likely to cause cancer under certain thresholds, and limiting your exposure to harmful substances goes a long way to reducing any associated risk.
Plus, the poor health outcomes observed among commuters – which does include higher rates of cancer – may stem from a combination of inactivity, obesity and shorter sleep that often come with lengthy commutes. These factors were not considered in this study.
Some people, however, have no option but to travel by car - or perhaps they are taxi drivers, whose job is to drive. To reduce their risk of exposure, more could be done during the car manufacturing process to substitute chemicals such as benzene and formaldehyde for less harmful alternatives, the researchers said.
"As people with long commutes are an already vulnerable sub-population, additional measures may need to be implemented in order to mitigate potential cancer risks associated with benzene and formaldehyde exposure," the duo wrote in their paper.
"There should be alternatives to these chemicals to achieve the same goals during vehicle manufacturing," added Volz. "If so, these should be used."
Improving access to public transport and bicycle networks could also give people more options to get to work while at the same time adding some incidental exercise to their commute and helping to alleviate traffic congestion that clogs up city roads.
The research was published in Environment International.