Read the full review here 

Why was the review conducted?
lobally, outdoor air pollution is a serious public health problem. In 2016, approximately 4 million deaths were attributable to air pollution, mostly from cardiovascular and respiratory diseases. Air pollution has also been linked to other health problems, like asthma. It is of much concern both in low‐ and middle‐income countries, where air quality may still be worsening, as well as in high‐income countries, where pollution levels have decreased over several decades.
Many different policies and programmes have been put into place to reduce air pollution; examples include vehicle restrictions to reduce traffic, fuel standards for cars, buses and other motorized transport, industrial regulations to limit pollution from factories, and the replacement of inefficient heating stoves with more efficient, cleaner burning stoves. So far, no review has investigated systematically whether these measures have impacted air pollution and health as intended.

What is the aim of this review?

We investigated whether measures put into place to reduce outdoor air pollution have actually reduced air pollution and improved health.

What were the main results of this review?

We found 42 studies evaluating a broad range of measures to reduce air pollution in different countries around the world, although most were from high‐income countries. Most aimed to reduce air pollution from cars and other vehicles. However, we also identified measures addressing heating and cooking, industry, or a combination of different sources.

We wanted to know whether these measures led to a reduction in the overall number of deaths, and in the number of deaths from cardiovascular and respiratory causes. We also investigated whether the measures led to fewer people going to hospitals for cardiovascular and respiratory problems. We also examined whether there were any changes in outdoor air quality, looking at different pollutants, such as particulate matter, fine particulate matter and other criteria pollutants.

Studies were very diverse with respect to the policies or programmes they assessed, the settings and contexts in which they were implemented, and the methods used to evaluate them.

The evidence we identified was of low and very low certainty, which means we cannot be very confident in the overall findings. Questions around certainty arose because of how studies were designed, conducted and analyzed. While some studies applied rigorous methods, others did not.

Overall, we observed mixed results across studies. Many studies observed no clear changes in health or air quality associated with the measures, while others did observe clear improvements. We identified very few studies that reported worsened health or air quality associated with the measures.

How do we interpret these results?

Differences in the studies make it difficult to draw general conclusions about whether the measures worked. Detecting changes in population health and air pollution levels is challenging, and assessing whether changes that occur are due to a specific measure is complex. Air pollution levels are changing constantly and often unpredictably due to weather and other factors, and other changes happening at the same time could also impact population health and air pollution. When regulations to limit industrial pollution are introduced, one must keep in mind that several other changes may be occurring in the background: an increase in traffic and an upgrade of residential heating systems, for example, or an economic downturn that leads to reduced pollution. It can sometimes take a long time before improvements in health become apparent. In interpreting the review’s findings it is important to remember that just because a study did not detect an improvement does not mean that there really was no improvement.

Further evaluations of measures to reduce outdoor air pollution in different countries, in particular in low‐ and middle‐income countries, are needed. Wherever possible, future evaluations should apply more reliable and standardized methods to analyze the data. This should help improve the quality of individual studies as well as our confidence in the findings across studies.

How up to date is this review?

This review includes studies up to 31 August 2016; any studies that were published after that date are not included in this review.

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