Review questions
1. What options are there for people with and without long-term breathing problems to reduce their exposure to outdoor air pollution?
2. Do these options have any impact on the health of people with long-term breathing problems?
Background to the question
Outdoor air pollution is a major problem. The World Health Organization (WHO) estimates that more than 90% of people live in places where air pollution is at harmful levels. It is thought that the average person living in Europe loses one year of life due to poor air quality.
Air pollution tends to have a bigger effect on people who already have breathing conditions, such as asthma and chronic obstructive pulmonary disease (COPD).
Some options to help reduce the effects of air pollution include wearing a mask that filters out pollution when outside, avoiding certain roads with a lot of traffic, or using air pollution alerts. It is not clear how well these options work, and there is also a chance that such options might have unwanted effects or be unpleasant.
We decided to do this piece of research after meeting a group of people with COPD in London. We asked them to tell us their most important questions about their health. Several group members wanted to know if there was any evidence about what they could do themselves to breathe less air pollution.
Study identification and selection
We searched multiple online databases for studies that tried different options, such as masks and air pollution alerts. We looked for studies in any language, published anywhere in the world and at any time, and also unpublished information. Two researchers looked at the lists of studies separately and then agreed on which ones we should include. We carried out our most recent search on 16 October 2020.
Study characteristics
We included any study which tried an individual-level intervention for reducing the amount of air pollution people were exposed to. By this we mean something that a person can do themselves, e.g. wearing a mask, or signing up to receive alerts about air pollution levels. We included studies in healthy adults and children, as well as people with long-term breathing conditions. The main measurements we were interested in were: measures of air pollution exposure; flare-ups of breathing conditions; hospital admissions; quality of life; and serious unwanted side-effects. Most studies were funded by a government or charity grant.
Key results
We found eleven studies to include in this review. The studies tried several different ways to reduce air pollution exposure: five studies used masks that filter out pollution, five studies used air pollution alerts and education, and one study tested a lower-level pollution cycle commute. The studies varied in size from 15 people to over 1000.
Because the studies were all so different, we could not combine the results statistically. We also found that most of the studies could not 'blind' participants or study personnel, which means the people involved in the studies knew whether they were receiving the option that was meant to reduce pollution exposure. This is important because knowing this might influence the way people behave.
Pollution-filtering mask and cycle-route studies
We found that masks and a lower-level pollution cycle route might have a small effect on measures that show you have been exposed to air pollution (e.g. blood pressure), but the results from the different studies were varied and we were very unsure. One study reported that people found breathing slightly more difficult while wearing a mask, but none of the other studies specifically recorded unwanted side-effects. People using a lower-level pollution cycle route had less irritation in their nose and throat, but it did not affect any other breathing symptoms.
Air quality alert studies
One study found that sending people alerts when the air quality is bad may increase the number of times they attend the emergency department or get admitted to hospital. But two other studies which looked at this did not find a clear difference between people who received the alerts and people who did not.
We found that in some studies people who received air pollution alerts and education about avoiding air pollution reported more 'preventative' behaviours, e.g. avoiding outdoor exercise when air quality was bad. But in other studies, the alerts did not appear to make much difference.
Another study reported that there was no clear difference in the breathing symptoms between those who received air pollution alerts and those who did not.
We have provided definitions of key words in a glossary (Table 1).
Bottom line
We did not find many studies to help us answer this question. The studies we found were quite different from one another so we were unable to combine them together to make a clearer picture. This means that we still cannot be sure what the best advice is to give to people who want to reduce the impact of air pollution in their day to day lives.
The lack of evidence and study diversity has limited the conclusions of this review. Using a mask or a lower-pollution cycle route may mitigate some of the physiological impacts from air pollution, but evidence was very uncertain. We found conflicting results for other outcomes, including health care usage, symptoms and adherence/behaviour change. We did not find evidence for adverse events.
Funders should consider commissioning larger, longer studies, using high-quality and well-described methods, recruiting participants with pre-existing respiratory conditions. Studies should report outcomes of importance to people with respiratory conditions, such as exacerbations, hospital admissions, quality of life and adverse events.
More than 90% of the global population lives in areas exceeding World Health Organization air quality limits. More than four million people each year are thought to die early due to air pollution, and poor air quality is thought to reduce an average European's life expectancy by one year. Individuals may be able to reduce health risks through interventions such as masks, behavioural changes and use of air quality alerts. To date, evidence is lacking about the efficacy and safety of such interventions for the general population and people with long-term respiratory conditions. This topic, and the review question relating to supporting evidence to avoid or lessen the effects of air pollution, emerged directly from a group of people with chronic obstructive pulmonary disease (COPD) in South London, UK.
1. To assess the efficacy, safety and acceptability of individual-level interventions that aim to help people with or without chronic respiratory conditions to reduce their exposure to outdoor air pollution.
2. To assess the efficacy, safety and acceptability of individual-level interventions that aim to help people with chronic respiratory conditions reduce the personal impact of outdoor air pollution and improve health outcomes.
We identified studies from the Cochrane Airways Trials Register, Cochrane Central Register of Controlled Trials, and other major databases. We did not restrict our searches by date, language or publication type and included a search of the grey literature (e.g. unpublished information). We conducted the most recent search on 16 October 2020.
We included randomised controlled trials (RCTs) and non-randomised studies (NRS) that included a comparison treatment arm, in adults and children that investigated the effectiveness of an individual-level intervention to reduce risks of outdoor air pollution. We included studies in healthy individuals and those in people with long-term respiratory conditions. We excluded studies which focused on non-respiratory long-term conditions, such as cardiovascular disease. We did not restrict eligibility of studies based on outcomes.
We used standard Cochrane methods. Two review authors independently selected trials for inclusion, extracted study characteristics and outcome data, and assessed risk of bias using the Cochrane Risk of Bias tool for RCTs and the Risk Of Bias In Non-randomised Studies - of Interventions (ROBINS-I) as appropriate. One review author entered data into the review; this was spot-checked by a second author. We planned to meta-analyse results from RCTs and NRS separately, using a random-effects model. This was not possible, so we presented evidence narratively. We assessed certainty of the evidence using the GRADE approach. Primary outcomes were: measures of air pollution exposure; exacerbation of respiratory conditions; hospital admissions; quality of life; and serious adverse events.
We identified 11 studies (3372 participants) meeting our inclusion criteria (10 RCTs and one NRS). Participants’ ages ranged from 18 to 74 years, and the duration of studies ranged from 24 hours to 104 weeks. Six cross-over studies recruited healthy adults and five parallel studies included either people with pre-existing conditions (three studies) or only pregnant women (two studies). Interventions included masks (e.g. an N95 mask designed to filter out airborne particles) (five studies), an alternative cycle route (one study), air quality alerts and education (five studies). Studies were set in Australia, China, Iran, the UK, and the USA.
Due to the diversity of study designs, populations, interventions and outcomes, we did not perform any meta-analyses and instead summarised results narratively. We judged both RCTs and the NRS to be at risk of bias from lack of blinding and lack of clarity regarding selection methods. Many studies did not provide a prepublished protocol or trial registration.
From five studies (184 participants), we found that masks or altered cycle routes may have little or no impact on physiological markers of air pollution exposure (e.g. blood pressure and heart rate variability), but we are very uncertain about this estimate using the GRADE approach. We found conflicting evidence regarding health care usage from three studies of air pollution alerts, with one non-randomised cross-over trial (35 participants) reporting an increase in emergency hospital attendances and admissions, but the other two randomised parallel trials (1553 participants) reporting little to no difference. We also gave the evidence for this outcome a very uncertain GRADE rating. None of our included trials reported respiratory exacerbations, quality of life or serious adverse events.
Secondary outcomes were not well reported, but indicated inconsistent impacts of air quality alerts and education interventions on adherence, with some trials reporting improvements in the intervention groups and others reporting little or no difference. Symptoms were reported by three trials, with one randomised cross-over trial (15 participants) reporting a small increase in breathing difficulties associated with the mask intervention, one non-randomised cross-over trial (35 participants) reporting reduced throat and nasal irritation in the lower-pollution cycle route group (but no clear difference in other respiratory symptoms), and another randomised parallel trial (519 participants) reporting no clear difference in symptoms between those who received a smog warning and those who did not.