'Safe' levels of air pollution could still be harmful

Monday December 9 2013

“EU air quality rules are still too lax to protect us from pollution,” The Independent reports. It says that air quality regulations may not be sufficient to protect people from harmful sooty particles in traffic and factory fumes.

Pollutants are measured using a system known as particulate matter or PM, based on the size of the individual element; this is measured in micrometres. As a general rule, the lower the PM, the more dangerous the pollutant is, as very small particles are more likely to bypass the body’s defences and potentially cause lung and heart problems.

The research the newspaper reported on, which combined the results of 22 European studies in 367,251 people, found a 7% increase in mortality with each 5 micrograms per cubic metre increase in particulate matter with a diameter of 2.5 micrometres (PM2.5).

This was even after taking into account various socioeconomic, health and lifestyle factors. The increased risk was found even below the recommended European safety threshold currently set for PM2.5 concentration.

These findings are troubling as they suggest (though do not prove) that exposure to previously regarded ‘safe’ levels of air pollution may still be harmful.

Further analyses of the link with specific causes of death are planned, and these should help researchers to show whether the excess deaths are from causes that could biologically be linked with these pollutants.

Where did the story come from?

The study was carried out by researchers from Utrecht University in the Netherlands and various other European institutions and was funded by the European Community’s Seventh Framework Programme.

The study was published in the peer-reviewed medical journal The Lancet.

The UK media’s reporting of the study was accurate.

What kind of research was this?

The researchers say that various studies, particularly from the US, have shown that long term exposure to air pollution with a mass concentration of particulate matter (PM) smaller than 10 micrometres (PM10) or 2.5 micrometres (PM2.5) in diameter can have an influence on mortality.

However, few European studies are said to have investigated PM concentration effects on health and mortality, although some have shown associations between nitrogen dioxide (NO2) or nitrogen oxides (NOx) in the air and mortality.

The current research was part of the European Study of Cohorts for Air Pollution Effects (ESCAPE). This is an on-going project looking at the effects of air pollution on health.

The researchers combined standardised exposure assessments for PM, NO2 and NOx to health data from 22 ongoing European cohort studies to look at the association between exposure to these air pollutants and mortality. The current analysis specifically looked at death from any natural-cause. Future analyses will look at death from specific causes.

What did the research involve?

The research included 22 cohort studies from 13 countries across Europe and featured 367,251 participants.

The researchers measured the levels of certain air pollutants in the regions where the participants lived, and then identified which participants died over an average of about 14 years.

They then looked at whether those exposed to higher levels of different air pollutants were more likely to die during the study than those exposed to lower levels.

The participants included were selected from the general population. Some samples covered large areas of the respective country, including large cities and surrounding smaller rural communities. Recruitment to the studies was mainly during the 1990s, and the studies varied in the number and age of participants, and which other health and lifestyle factors were assessed.

The length of follow-up in the studies varied from six to 18 years (average 13.9 years). 

Participant deaths were identified through the countries’ death registers.

Deaths from natural-causes were identified on the basis of the underlying cause of death recorded on death certificates.

Causes of death not conceivably related to air pollution exposure (such as injury, accidents or suicide) were excluded.

The researchers estimated concentrations of air pollutants at the participants’ home addresses (at the start of the study) using a standard procedure. Air pollution was monitored between October 2008 and May 2011, and an average annual concentration of the pollutants PM10, PM2.5, NO2 and NOx was calculated.

As well as looking at pollutant concentrations, the researchers measured traffic intensity on the nearest road (vehicles per day), and total traffic load (intensity multiplied by length) on all major roads within a 100 metre buffer area. This was done as another way of estimating exposure to air pollution.

The researchers compared the risk of death over time between participants exposed to different levels of air pollutants. They took into account other factors that could affect risk of death (potential confounders). These were assessed based on questionnaires completed by participants at the start of the studies and included:

  • year of enrolment into the study
  • gender
  • smoking status, smoking frequency and duration, and exposure to environmental tobacco smoke
  • fruit and vegetable intake
  • alcohol consumption
  • body mass index (BMI)
  • educational level, occupational class and employment status
  • marital status
  • socioeconomic status of the areas where the participants lived

What were the basic results?

During the study, 29,076 of the 367,251 participants died (8%). The average NO2 concentration ranged from 5.2 micrograms per cubic metre to 59.8 micrograms per cubic metre. The average PM2.5 concentration varied from 6.6 micrograms per cubic metre to 31.0 micrograms per cubic metre.

The pooled results of all cohorts revealed that each 5 microgram per cubic metre increase in PM2.5 concentration was associated with a 7% increase in mortality risk (hazard ratio [HR] 1.07, 95% confidence interval [CI] 1.02 to 1.13).

When looking at the association between mortality and different concentrations of PM2.5, there was a borderline significant increased risk of mortality when people were exposed to a concentration below the European annual mean limit of 25 micrograms per cubic metre (HR 1.06, 95% CI 1.00 to 1.12), and below 20 micrograms per cubic metre (HR 1.07, 95% CI 1.01 to 1.13).

At lower thresholds (15 or 10 micrograms per cubic metre) mortality was still raised, but this increase did not reach statistical significance.

There was no statistically significant association between mortality and increasing concentrations of NO2, NOx, PM10, traffic intensity on the nearest road or traffic intensity on major roads within 100 metres.

How did the researchers interpret the results?

The researchers conclude that “long-term exposure to fine particulate air pollution was associated with natural-cause mortality, even within concentration ranges well below the present European annual mean limit value”.


This is a valuable study combining the results of 22 European cohorts looking at the association between air pollutant concentration and overall mortality.

The analysis benefits from its large size, long duration of follow-up (average 13.9 years), ability to adjust for various potential confounders, and standardised measures of air pollution and mortality across studies.

The study found no association between mortality and average annual concentrations of PM10, NO2 and NOx or traffic intensity on nearest roads and major roads. However, they did find a 7% increase in mortality with each 5 micrograms per cubic metre increase in PM2.5 concentration. This link remained even after taking into account various socioeconomic, health and lifestyle factors. The findings are reported to be similar to those of a recent pooling of studies looking at the effect of PM2.5 on death from any cause.

Of particular note the researchers found significant associations with mortality at the European annual mean limit for PM2.5 of 25 micrograms per cubic metre. The World Health Organization’s air quality guideline suggests an annual mean limit for PM2.5 of 10 micrograms per cubic metre, and the authors suggest that their findings imply that moving towards this level could lead to health benefits.

The researchers note that when comparing the cohorts, they found that the variation in PM2.5 concentration was not solely related to traffic variables, but also seemed to vary according to population density, industrial sources, urban green space, and elevation above sea level.

Although measurement of air pollution was standardised, it is difficult to accurately measure how much pollution an individual is exposed to. The estimates in the current study were based only on the participants’ home addresses at the start of the study – these could change, and they could have different levels of exposure at work. More accurate measurements are likely to be difficult to obtain.

And despite adjustment for the measured confounders, it is still difficult to be certain that the PM2.5 concentration was itself the factor directly responsible for the increase in overall mortality risk.

However, the fact that this link was found consistently across different countries does seem to suggest it is a robust finding.

The findings suggest that even low air concentrations of fine particulate matter seem to be associated with mortality. This highlights the importance of continuing to work towards reducing levels of air pollution.

Analysis by Bazian
Edited by NHS Choices