A diesel “WHO dunnit”

The World Health Organization (WHO) has ruled that diesel exhaust fumes cause cancer. This could be an opportunity for Autogas, which does not.

On 12 June 2012, the WHO’s International Agency for Research on Cancer (IARC) made it official. After a week-long review and discussion of evidence by 27 experts, 10 observers and 15 members of IARC staff, IARC concluded that diesel exhaust is carcinogenic to humans. In other words, breathing diesel tailpipe emissions causes cancer. Not maybe, not possibly, but – given enough exposure – definitely.

The decision, based on exhaustive research, was a long time coming. Its implications for diesel are dramatic. Standards for ambient concentrations of and exposure to diesel exhaust, say researchers, should be tightened. In practice, this means two measures. First, at repair stations for diesel cars and trucks, ventilation must be high and maintenance workers might be required to wear masks. Second, in polluted urban areas, more incentives must be given for motorists to switch from diesel to other fuels. Gasoline is not an option, because the same IARC working group has ruled it a possible carcinogen. So the door is open to alternatives, including Autogas.

How do they know?

Deciding whether a material is carcinogenic is time- and labour-intensive, usually involving two study types. One involves animal testing. After exposure and death, the subjects are dissected to see if cancer was caused and how it spread. The other type of study involves humans; of course, they are not tested, rather they are observed. The aim is to monitor a large sample over many years. For diesel cancer, studies have been made of three main groups: truck drivers, miners and railroad workers.

Research in this area is truly massive. An estimated $5-6 billion a year is spent worldwide on cancer science. A recent search on ‘diesel cancer’ in the archives of Science, the leading scientific journal in the world, returned 30 000 references.

Suspicion of diesel cancer goes back a long way. The first links – quite similar to those between cigarettes and cancer – were found in the mid-1950s. But it was not until the late 1970s or early 1980s that the connection became much more obvious. At that point, regulators began to fund many more, big studies. And they began to reach conclusions. In 1998 the US State of California declared particle matter in diesel exhaust to be carcinogenic. The US Environmental Protection Agency says diesel exhaust is a likely carcinogen; Germany’s Environment Agency says it is a suspected carcinogen. And other regulators are of similar opinions.

Who’s WHO

This all led up to WHO’s decision this past June, which, given all the research and precedents, is not really surprising. In fact, it would have been surprising had the WHO not reached this conclusion.

Why did it not happen earlier? A group of coal-mine owners, the Mining Awareness Resource Group, held up the ruling for something like a decade. In 1992, a $12-million study of coal miners was launched by two US government agencies, the National Institute for Occupational Safety and Health (NIOSH) and the National Cancer Institute (NCI). This ‘Diesel Exhaust in Miners Study’ tracked thousands of people over several decades (they were exposed to diesel fumes while underground.) Already in the mid-1990s, the Mining Awareness group began to challenge the study in the US courts. The legal battle lasted until earlier this year, when NIOSH and the NCI were finally able to publish their results, which led to WHO’s decision in June.

Many scientists were and are upset about the delay and the mine owners’ challenge. The miners’ lawyer, Henry Chajet of Patton Boggs LLP, wrote to scientific journals, urging them not to publish results of the study. Some journal editors claim that Chajet even threatened them. To be fair, Chajet had at least one valid objection. He said the study does not represent current practice; mines today give lower exposures to diesel exhaust, and so the NIOSH/NCI study, in some respects, is outdated. At the same time, Chajet did not challenge the basic premise: exposure to diesel exhaust causes cancer.

What’s what

That happens through two main events. One is that people breathe exhaust into their lungs. Particle size is key: those under 2.5 microns in diameter can go really deep. If they contain a carcinogen, this attacks DNA. And that can cause lung cancer. It also can attack the bladder. As carcinogens in the lungs are broken down, those products can also be carcinogenic. They are removed by the kidneys to urine, which of course collects in the bladder – bingo.

Carcinogens in diesel exhaust are similar to those in cigarette smoke. These are combustion products from some of diesel fuel’s more complicated components: aromatics and polycyclics. LPG, by contrast, is chemically far simpler. Combustion of it generates some particles and even, conceivably, some carcinogenic ones. But the likelihood of this happening and the volumes involved are far lower. Its safety is validated by a critical fact: LPG is regularly used indoors without a flue.

One possible answer to diesel’s dilemma is particle filters. These are now mandatory for new cars in the European Union as well for many heavy vehicles. Filters remove more than 99% by weight of particle matter from a diesel exhaust. However, filters do not necessarily take out 99% of particles. Indeed, recent research shows that filters might actually increase the number of tiny particles: precisely the ones with highest cancer risk. Conclusions are tentative, and they are affected by fuel composition, engine conditions and sampling conditions. Nonetheless, when people blithely say that particle filters will solve diesel cancer (and surely they will), this pat answer should be rejected.

So, WHO’s decision is another reason for regulators to act. Experts are calling for stringent regulation of diesel exhaust. For instance, Dr Lesley Rushton of Imperial College London’s Department of Epidemiology and Biostatistics wrote in the June 2012 issue of the Journal of the National Cancer Institute that there should be better ventilation, engine modifications and even mandatory mask-wearing for people such as miners and truck drivers. This sounds reasonable. But so far, there has been no clamour for switching from diesel to Autogas. In fleets and in cities, where exposure to diesel exhaust can be high, this could be an excellent antidote to diesel cancer. Now it’s up to you to let more people know.

Eric Johnson can be reached by email at: ejohnson@ecosite.co.uk