Wednesday July 10 2013
It is estimated that there are 40 million cars in the UK
Air pollution, chiefly from traffic exhaust fumes in cities, is having a serious and sometimes fatal effect on health, The Guardian reports.
This follows the publication of two studies in The Lancet that examined the effects of short- and long-term exposure to pollutants on the risk of developing lung cancer or heart failure.
These were both well conducted studies that collected a large body of evidence. Researchers looked at observational studies examining the impact of air pollution on long-term health outcomes.
The lung cancer study pooled the results of 17 studies. It found that increased concentrations of particulate matter with diameter greater than 10 micrometers was associated with increased risk of lung cancer. Particulate matter is a pollutant made up of a mixture of liquid droplets and solid particles found in the air and is produced by sources including car exhausts.
The heart failure study, which pooled the results of 35 observational studies, also found an association between increased concentrations of particulate matter with diameter greater than 2.5 micrometers and risk of heart failure.
However, it is important to bear in mind the limitations of these studies. These include the possible influence of other unmeasured confounding factors, and the possibility of inaccurate estimation of pollutant exposure.
Nevertheless, these are important findings. Air pollution is already being targeted for reduction by governments and organisations such as the World Health Organization with the aim of improving lung and heart health.
Where did the story come from?
Both studies were published in the peer reviewed medical journal The Lancet. The lung cancer study was conducted by researchers from the Danish Cancer Society Research Center, Copenhagen, Denmark, and other research institutions in the Netherlands, Greece, Italy and Germany. Funding was provided by the European Community.
The heart failure study was conducted by researchers from the University of Edinburgh and the Public Health Foundation of India, New Delhi, and was funded by the British Heart Foundation.
The UK media reported the findings of the studies accurately and some news sources included useful quotes from independent experts.
What kind of research was this?
Lung cancer study
The lung cancer study used the pooled results of 17 cohort studies conducted in nine European countries. The researchers say that while smoking is a firmly established risk factor for lung cancer, occupational exposures and environmental factors are also recognised risk factors.
Air pollution, specifically particulate matter containing chemicals called absorbed polycyclic aromatic hydrocarbons and other chemicals, can damage DNA. And damage to DNA is believed to increase risk of lung cancer. Previous research is said to have observed associations between air pollution in both smokers and people who have never smoked, and among people with low fruit consumption.
The current study, called the European Study of Cohorts for Air Pollution Effects (ESCAPE), analysed the results of 17 cohorts with the aim of addressing the questions of:
- whether air pollution (specifically particulate matter) at the place of residence is associated with risk of lung cancer
- whether the association between air pollution and lung cancer is stronger for non-smokers and people with low fruit intake
- whether the association with air pollution is stronger for any of the different types of lung cancer – squamous cell (the most common cancer, often observed in smokers); adenocarcinoma (the second most common, which develops from the mucus-producing cells of the lungs) and carcinomas – than for all lung cancers combined
Heart failure study
The heart failure study had a slightly different study design. Previously, exposure to air pollution has been associated with risk of heart attack. The heart failure study aimed to see whether there is also a link with heart failure. To do this the researchers conducted a systematic review. They attempted to identify all studies examining the association between environmental increases in particulate and gaseous (carbon monoxide, sulphur dioxide, nitrogen dioxide, ozone) pollutants and deaths and hospital admissions due to heart failure.
What did the research involve?
Lung cancer study
The 17 cohort studies were conducted in nine European countries where air pollution had been measured at different locations. The studies also included information on the number of new lung cancer diagnoses, and information on important confounders had also been collected.
The main outcome was diagnosis of any type of primary lung cancer (that is cancer that had originated in the lung – not metastatic cancer that had spread to the lung from a cancer elsewhere in the body). This was coded according to an internationally agreed classification system (the International Statistical Classification of Diseases and Related Health Problems, 10th edition or ICD-10). Secondary outcomes were the specific type of lung cancer diagnosed.
During October 2008 to April 2011 air pollutant concentrations at the home addresses of participants were measured during different seasons. They included measurement of:
- particulate matter with a width (aerodynamic diameter) of less than 10 micrometers (PM10), and particulate matter with a width of less than 2.5 micrometers (PM2.5)
- soot and black carbon
- nitrogen oxides (NOx)
- nitrogen dioxide (NO2)
They also looked at other factors associated with pollutants, such as density of traffic, roads and buildings.
The researchers followed up all cohort participants from the time of study enrolment to the time of lung cancer diagnosis, death, emigration or the end of study follow-up. They had excluded participants who already had past diagnosis of cancer at the time of study enrolment.
Statistical models were created to look at the association between air pollution exposure and risk of lung cancer diagnoses. The models were adjusted for potential confounders, including:
- smoking status (including intensity and duration of smoking)
- environmental smoke exposure
- socioeconomic status
- fruit intake
Heart failure study
The researchers of this study performed a search across five literature databases to find observational studies examining the association between heart failure hospitalisations and deaths and incremental increases in PM2.5 and carbon monoxide, sulphur dioxide, nitrogen dioxide and ozone.
Thirty-five studies were eligible for inclusion. They pooled the adjusted risk estimates (adjusted for whatever confounders each study had measured) for each study to estimate the risk associated with each pollutant.
What were the basic results?
Lung cancer study
The 17 cohort studies in nine countries included 312,944 people who had an average age at the time of study enrolment of between 43 and 73 years. There was an average follow-up of 12.8 years across cohorts, during which time 2,095 new lung cancers developed. The number of lung cancer cases varied between the countries, with Danish and Austrian cohorts accounting for more than half of the total lung cancer cases. The cohort areas were also said to represent a wide range of air pollution concentrations. For example, average air pollution levels were up to 12 times higher in some southern European areas compared with levels in some northern European areas.
With full adjustment for all measured confounders, the pooled results from the cohorts showed that each increase in concentration of PM10 (each 10 micrometers/m3 increase) led to a corresponding increase in lung cancer risk (hazard ratio 1.22, 95% confidence interval [CI] 1.03 to 1.45).
However, for the other types of pollutant measured (PM2.5, soot and black carbon, NOx, NO2) there was no significant increase in lung cancer risk.
Also, traffic density at nearest road and traffic load on major roads within 100m, were not significantly associated with risk of lung cancer.
Looking at specific types of lung cancer, increases in concentrations of PM10 and PM2.5 were both associated with increased risk of adenocarcinoma. But, by contrast, neither was significantly associated with increased risk of squamous cell cancer.
Heart failure study
The researchers of the heart failure study found that increases in the following were associated with significantly increased risk of heart failure hospitalisation or death:
- increase in carbon monoxide of one part per million: 3.52% increase in risk (95% CI 2.52 to 4.54% increase)
- increase in sulphur dioxide of 10 parts per billion: 2.36% increase in risk (95% CI 1.35 to 3.38% increase)
- increase in nitrogen dioxide of 10 parts per billion: 1.70% increase in risk (95% CI 1.25 to 2.16% increase)
- increase in PM2.5 of 10 micrometers/m³: 2.12% increase in risk (95% CI 1.42 to 2.82% increase)
- increase in PM10 of 10 micrometers/m³: 1.63% increase in risk (95% CI 1.20 to 2.82% increase)
There was no significant association between ozone levels and heart failure risk.
How did the researchers interpret the results?
Lung cancer study
The researchers of the lung cancer study conclude that “particulate matter air pollution contributes to lung cancer incidence in Europe”.
Heart failure study
The researchers of the heart failure study conclude that air pollution has a close association with hospitalisations and deaths due to heart failure. Although they acknowledge that more studies are required, they say that “air pollution is a pervasive public health issue with major cardiovascular and health economic consequences, and it should remain a key target for global health policy”.
These well conducted studies that have collected a large body of evidence found associations with increasing levels of environmental pollutants and risk of both lung cancer, and hospitalisations and deaths due to heart failure.
The lung cancer study analysed data from more than 300,000 people from a range of European countries and, importantly, took into account people’s detailed smoking history.
It found a significant association between increased concentrations of one type of particulate matter in the air (PM10) and risk of any type of lung cancer, with non-significant associations for the other pollutants measured.
Further analysis by cancer type found that both PM10 particulate matter and smaller particulate matter (PM2.5) were significantly associated with adenocarcinoma, a type of lung cancer that is becoming increasingly common.
The second Lancet systematic review found an association between PM2.5 and a number of other air pollutants and heart failure.
However, there are some limitations of these studies to bear in mind. The lung cancer study adjusted for a wide range of potential confounders, including smoking history. However, they could not take into account change in smoking habits during follow-up. There were also some other potential confounding factors that they did not have data on, such as previous lung diseases. They also acknowledge that estimation of exposure at the home address of each participant may not be entirely accurate.
As the researchers of this study also say, pollutants are part of complex mixtures of chemicals, so it is often difficult to say which particular chemicals are having the effect.
In the heart failure review, the individual observational studies that were pooled were of varying quality. They differed in terms of population demographics and characteristics and sample size, and there was variable accuracy in the regional monitoring of air pollutants. This means that exposures may have been misclassified. Also, measuring single pollutants does not take into account the potential effects of various pollutants combined. There is also the possibility for incorrect coding of heart failure deaths and hospitalisations, and the results could not take into account multiple hospitalisations for the same person.
Nevertheless, these are important findings that provide additional support for a link between certain air pollutants and the risk of lung cancer. They also suggest a link with heart failure. Air pollution is already being targeted for reduction by governments and organisations such as the World Health Organization, with the aim of improving lung and heart health.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.