Diesel emissions – an emerging public health issue?

In early September, German car giants Volkswagen admitted they’d been covertly modifying their diesel-powered cars with so-called ‘defeat devices’, in an attempt to get around national and international emissions regulations.

This caused outrage around the world, much of which was directed at the potential environmental repercussions of the resulting extra NOx pollution – estimated by some to be in the region of 1 million tonnes.

But are rising levels of diesel emissions also leading us towards substantial public health problems too? As we wrote about back in 2012, diesel emissions are a known carcinogen, classified by the International Agency for Research on Cancer as a ‘group 1’ carcinogen which causes lung cancer.

Today, in the Annals of Oncology, a group of leading cancer researchers – including Cancer Research UK’s Professor Charlie Swanton – together with high-profile economics journalist Robert Peston, argue for urgent investigation into the matter.

They write:

Whilst the size of the effect is small when compared to smoking, the number of people at risk to low exposure levels over prolonged periods of time is large, particularly when the inexorable rise in diesel vehicles over the last decade is considered…

The impact of defeat devices in some diesel vehicles may not be simply an economic problem with no public health consequences.

So, are they right to be concerned? Let’s have a quick look at what we do – and don’t – know.

Lung cancer stats – a complex picture

As it stands, about nine in every 10 lung cancers in the UK are linked to smoking. And lung cancer is the 2^nd most common cancer in the UK after breast cancer.

And if you take out every case of cancer (any type, not just lung) in the UK thought to be due to smoking , lung cancer drops from 2^nd most common to 12^th*.  But that still means lung cancer would affect some 6,260 people every year in the UK – still a significant public health issue, especially given the poor survival in lung cancer overall compared with other cancers.

Thanks to continued action against tobacco, the country’s overall lung cancer rates are on the way down. But in the lung cancer type most common in non-smokers – adenocarcinoma – the number of cases seems to be on the up:

But this is just indirect evidence that there may be something going on. Is there any evidence that lung cancer in non-smokers is on the rise?

Unfortunately it’s really tricky to tell – particularly since even non-smokers are exposed to second-hand smoke (much more so in the past than today, thanks to the public smoking ban).

So while the trends in lung adenocarcinoma don’t constitute ‘proof’ that lung cancer caused by factors other than smoking is on the increase, it does point to something underpinning these long-term trends that we do need to understand.

What about pollution levels?

It’s here that things get a bit tricky. Most of the data available tends to suggest that, like lung cancer rates, air pollution levels are generally falling too.

But this, too, could hide a number of other trends. The ‘nasties’ in diesel emissions are thought to be microscopic particles known as PM2.5 – and proper UK records for these only began to be collected in 2010.

This is far too short a time to confidently link pollution rates with lung cancer rates. Again, there’s a real lack of good quality data to rely on.

What about biology?

Professor David Phillips: “quietly optimistic” of progress in finding causes of lung cancer in non-smokers.

Of course, the answer to unknowns and uncertainty is research – both to fill the gaps in the data, and also on the underlying biology of lung cancer in non-smokers. This is something the authors of the editorial urgently call for – and it’s here that, thanks to recent advances in DNA analysis, some real progress is on the horizon.

Professor David Phillips is Cancer Research UK’s expert in carcinogenesis, and he’s currently involved in a project called the Collaboration on Mutational Signatures, or COMSIG, funded by the Wellcome Trust.

Phillips’s team is blasting lab-grown cells with a range of cancer-causing chemicals, allowing them to grow, then working with experts at the Wellcome Trust Sanger Institute to map out their entire genomes, looking for the fingerprints of damage from each chemical.

It’s an approach that’s already yielded some groundbreaking results – two years ago the Sanger team identified no fewer than 12 separate signatures, linked to things like tobacco, UV light, and other cancer-causing processes.

But the really clever bit in the COMSIG project comes after a carcinogen’s fingerprint has been identified. “We’ll then be able to scour public databases of cancer DNA such as the Cancer Genome Atlas, looking for examples of these fingerprints in their genes,” he says.

“One group of chemicals we’re focusing on are called nitro-PAHs, which we think look like the most likely cancer-causing agents carried into the lungs on PM2.5 diesel particles,” Phillips told us.

“Nitro-PAHs are a whole family of chemicals, some of which are more potent at causing cancer in the lab than others. So we don’t really know which one is the culprit in diesel emissions.”

Given that the DNA databases contain thousands of samples from non-smokers who developed lung cancer, the chances are high that they’ll find something. “I’m quietly optimistic, although in research you never know,” says Phillips.

What next?

So where does that leave us? The murky business of the negative health impacts of defeat devices in diesel cars has – as the experts writing today have highlighted – shone a welcome spotlight on three gaps in current knowledge:

• Rates of non-smoking-related lung cancer;
• pollution rates, particularly of PM2.5 particles;
• the human impact of environmental carcinogens.

But amidst the questions one thing is certain – we mustn’t take our eyes off the immediate goal: reducing smoking rates.

Apart from lung cancer, tobacco causes at least 13 other types of cancer, leading to around 100,000 premature deaths in the UK every year. The impact of tobacco’s lethal legacy is known and it will never be off the hook, even while we keep making strides in our understanding of causes of all forms of cancer.

Henry

*calculated by applying Parkin et al (2011) to 2011 UK cancer incidence statistics.

Reference

• Swanton, C., Boffetta, P., Peston, R., & Soria, J. (2015). Environmental Emissions, Public Health and Lung Cancer Risk Annals of Oncology DOI: 10.1093/annonc/mdv579