The link between air pollution and cancer

Air pollution can contribute to cancer

There have been several stories in the media today about air pollution causing cancer. These have been triggered by an announcement from the World Health Organisation’s International Agency for Research on Cancer (IARC) about their latest analysis of the evidence on outdoor air pollution.

Though some media reports have given a reasonable overall picture, there have also been some more misleading headlines that air pollution is the “leading cause of lung cancer” (it’s not, smoking is).

The evidence that outdoor air pollution increases the risk of cancer has been growing, and IARC had already classified diesel engine exhaust as well as some specific substances found within air pollution as causes of cancer.

So while this news isn’t surprising, it highlights the need for action on air pollution.

So how big are the risks for us in the UK? How does air pollution rank alongside other causes of cancer? And what can we do about it?

What makes up air pollution?

But first things first, what exactly do we mean by air pollution?

The reason air pollution is so hard for researchers to study is that it’s a complicated mixture of many different things. And the exact contents of that mixture vary widely, depending on whether there are any sources of pollution nearby, where in the world you are, the time of year and even the weather.

The main sources of air pollution include transport, industry, fossil fuel power stations, farming and fuels people use to cook and heat their homes. Some pollutants, North African desert dust for example, are natural.

Most of the research into the health effects of air pollution relates to a few substances that tend to be part of air quality regulations. This means their levels are measured in many places around the world, so scientists can look for links between the amounts people were exposed to and whether they developed cancer.

Particulate matter (PM) is the type of air pollution studied most, so the IARC had enough data to look at PM separately and conclude that PM is also a group 1 substance and causes cancer in people. PM is usually classified by its size – PM10 and PM2.5 are solid particles in the air that are smaller than, respectively, 10 and 2.5 millionths of a metre across.

Other common forms of outdoor air pollution include:

• Nitrogen dioxide
• Sulphur dioxide
• Ozone gas
• Carbon monoxide
• Polycyclic aromatic hydrocarbons (PAHs), which are a large family of chemicals, some of which have been individually linked to cancer. There are over 500 types of PAH that can be found in air pollution.

How good is the evidence?

IARC assessments are valuable because they’re based on the knowledge of a panel of experts from around the world. This panel reviews all the available evidence – this latest evaluation pulled together over 1,000 scientific papers and included studies from five continents.

The conclusion is that the evidence is now strong enough to say that outdoor air pollution is a ‘Category 1’ – or definite – cause of cancer.

This is particularly useful in the case of air pollution because the evidence generally has been mixed or difficult to interpret. Part of the reason is because the effects of air pollution on people are very difficult to measure. Individuals can live and work in places with very different pollution levels, and travel between them in different ways.

And there have been a lot of problems with the design of research on air pollution. For the most reliable results, studies also need to take account of a whole range of things that could also have an effect on lung cancer – most importantly whether or not people smoke, or have smoked in the past.

And there are the usual things that make for better studies too: for example, ideally this type of research should include lots of people and follow what happens to them over a long time. And for the findings to be most useful to us in the UK, it’s better to look at studies that take place in populations that are similar to our own.

As well as looking at epidemiological evidence, which tells us what happens in people exposed to air pollution, the IARC evaluation considered the evidence from studies on animals and cells in the lab. This type of evidence isn’t enough by itself to prove a link, but taken together with studies in people it helps strengthen the case.

Taking all this evidence into account, and with the above limitations in mind, IARC now believes outdoor air pollution should be considered a cancer-causing substance – a carcinogen.

How big is the risk?

But the chance of any one person in the UK developing lung cancer specifically due to air pollution is pretty small.

As with most cancer risk factors, the risk is higher the more someone is exposed to the substance. Air pollution is a complicated mixture that can contain many components, and different people will have been exposed to different amounts depending on their lifestyle. But it’s unlikely that in the UK many people would be exposed to high enough levels to make any noticeable difference to their risk of lung cancer.

It’s important that people keep the risk from air pollution in perspective. Although air pollution increases the risk of developing lung cancer by a small amount, other things have a much bigger effect on our risk, particularly smoking.

How does the risk compare with smoking?

Being a smoker poses a much bigger risk to health than air pollution. Long-term smokers can have 20 times the risk of lung cancer compared to people who have never smoked. And research has shown that more than eight out of 10 lung cancers in the UK are caused by smoking.

If you smoke, tobacco will have a much bigger impact on your risk of cancer than air pollution will. And the best thing you can do for your health is to quit – studies have shown that even long term smokers who quit in their 60s gain valuable years of life.

What about things like obesity, alcohol and diet?

On an individual level, the chance of developing lung cancer due to air pollution is pretty small. But small effects over large populations matter. In other words, because air pollution potentially affects all of us, it is an important cause of lung cancer across the whole population.

And air pollution has also been linked to heart disease as well as asthma attacks and other lung conditions.

The WHO’s major Global Burden of Disease project last year published an analysis of the most important risk factors that contribute to disease around the world.

This includes cancer, as well as conditions like heart disease and diabetes. The position in the list reflects both how much something increases the risk of disease and also how many people are affected. For the area of Western Europe tobacco, unsurprisingly, topped the list as the biggest contributor to disease in the region. And air pollution came in at number 11.

Between tobacco and air pollution, the WHO list several key factors related to lifestyle that we know increase the risk of cancer. High body mass indexes (BMIs), alcohol and low physical activity respectively rank 3rd, 4th and 5th as causes of disease within Western Europe.

So although air pollution can increase the risk of cancer and other diseases, overall sticking to a healthy lifestyle is still an important part of reducing the risk of cancer.

And, importantly, our lifestyles are within our own control.

And we wouldn’t want worries about air pollution to put people off walking and cycling. A really important way we can help cut the risk from air pollution is to create less of it. People choosing to use their cars less often could make a big difference to levels of air pollution. And, as we’ve already mentioned, being active is also a great way to improve your health and reduce your risk of cancer.

Some studies have found that when they modeled the effects of more people in a city choosing active transport options, like walking and cycling, overall people’s health improved.

How can we tackle air pollution?

Air pollution is “one of the classic problems of public health”, according to Dr Dana Loomis of IARC, speaking at the press conference. Tackling air pollution requires leadership and action at both national and local level.

But there are some things we can try ourselves to reduce the levels of air pollution we’re exposed to. In general, if you’re walking, running or cycling try to choose a route that uses smaller, less busy roads. For some areas, the Walkit website has a “low pollution” option. Apart from anything else, your walk is likely to be a more pleasant experience on quieter streets.

This announcement should act as a stepping stone for national and even international action to address air pollution. We want the government and other relevant authorities to introduce measures to reduce air pollution to levels within EU limits to protect people’s health.

Reference

Images courtesy of Tony Oxborrow, via Flickr.