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expert reaction to data published by the national air quality testing services on indoor air pollution

Reactions to data published by the National Air Quality Testing Services on indoor air pollution.


Dr Hugh Coe, Professor of Atmospheric Composition, University of Manchester, said: 

“This research highlights the importance of indoor as well as outdoor pollution. Though the work has not been published and has not been through the rigours of the peer review process it demonstrates very clearly that many activities inside the home such as cooking and the use of wood burning stoves are important sources of ultrafine particles.  Particles of similar size are present in outdoor air from motor vehicles and other sources but these measurements show this importance of indoor emissions on exposure.  As this work clearly demonstrates we need more information on this potentially important contribution to the particulate in the air we breathe.”


Prof Ian Colbeck, Professor of Environmental Science, University of Essex, said:

 “The fact that indoor air pollution can be much higher than that outdoors has been known for many years. There are numerous peer reviewed papers on the topic, all covering a much longer time period and more houses than this study. Indoor concentrations will vary throughout the week as ventilation rates vary along with the activities of the occupants.

“The link between ambient particulate matter, such as that produced by diesels, and health is well documented. Fine particles are associated with around 29,000 deaths per year in the UK.

“There are many indoor sources of particulate matter; their impact on health depends upon a range of factors including their physical properties (e.g. size, shape, and surface area) as well as chemical composition and microbiology. Overall the toxicity of indoor particles is very complex and may be quite different to those outdoors. In the absence of full evidence on the potential toxic effects and in accordance with the precautionary principle, reducing the numbers of all types of particles may lead to better health.

“In the coming months expect to see much greater attention being paid to indoor air. NICE will be publishing guidelines on a range of actions people can take to reduce their exposure to indoor air pollution at home. The Royal College of Paediatrics and Child Health and Royal College of Physicians are due to report on the impact of indoor air pollution on children and young people’s health in the UK.”


Professor John Gulliver, Deputy Director of the Centre for Environmental Health and Sustainability, University of Leicester, said:

“When looking at this data it is important to question how many measurements were taken and in how many dwellings in each of the areas investigated. Some dwellings, perhaps not included in this studymay have lower levels of indoor UFP (ultra fine particles) than outdoors because they are well ventilated, and the only major source of UFP is cooking, and the type of cooking, temperature, and type of oil generates lower numbers of UFP (and other pollutants), so a large sample size for work like this is important. 

In response to the findings listed in the press release:

  • Peaks of this type of indoor air pollution occur as a result of everyday activities, particularly cooking and using a woodburner.

“The limited evidence shows that cooking is the major contributor to elevated indoor levels of UFP particle count in dwellings.”

  • Our data also shows that outdoor air pollution adds to indoor air pollution, creating a build up of pollution in the home with pollution peaks taking longer than outdoors to disperse. The monitoring also identified that outdoor air pollution spikes created an increase in indoor air pollution.

“Agree, outdoor air pollution may add to indoor air pollution, but also vice versa.”

  • Ultrafine particle pollution levels were on average 3.5 times higher inside than outside, peaking at 560 times outdoor air pollution. 

“It is not clear whether the UFP levels were measured by particle number count. Presumably UFP was measured using a portable UFP sensor, and it is likely particle number count, but it would be good to get confirmation on this. Using a UFP particle number counter I have seen short-term (seconds up to about a minute) peaks 100-200 times higher indoors than outdoor, mostly related to cooking. The claim of 560 times higher seems plausible but these may be very short-term spikes. We don’t know if peaks (or amplitude) is more or no more important than the average over hours/days for health.

  • The air pollution monitored consisted of ultrafine particles (UFPs)[1]. According to a leading expert, ultrafine particles have the potential to have greater health impacts than PM10 or PM2.5 pollutants because they are smaller and evidence suggests they can be more easily absorbed into the body.

“Yes, because of their size they have potential to be more mobile inside the body and translocate from the lung to other organsOwing to their small size fraction, UFPs contain a high surface area to–mass ratio, giving them a high potential for translocation and interaction with tissue, resulting in oxidative stress and inflammation within extrapulmonary organs(HEI2013; Stoneetal.2016). The bulk [albeit not all (Jordakievaetal.2018)] of experimental animal and human studies have reported that UFP exposure is associated with atherosclerotic plaque formation, oxidative stress, increased inflammatory and procoagulant biomarkers, reduced coronary circulation, elevated blood pressure, and autonomic imbalance, suggesting that UFP exposure may play an important role in cardiovascular health(Aguileraetal.2016; Baietal.2018; Chungetal.2015; Keebaughetal.2015; Laneetal.2016; Liuetal.2018).


Prof Alastair Lewis, Professor of Atmospheric Chemistry, National Centre for Atmospheric Science, University of York, said:

“This isn’t a peer-reviewed scientific study, rather they are short one day snapshot experiments conducted by an NGO, supported by an air quality consultancy using fairly standard equipment. One doesn’t want to be too down on this, since I’m sure it has been undertaken with good intentions, but it’s really not particularly robust.  It is potentially misleading, since it is focusing attention on a few short transient peaks in indoor PM number during the day. Outdoor air quality is typically considered over annual timescales, smoothing out day to day and weather related variability. So to get a truly meaningful comparison of inside exposure compared to outside would need a much much longer period of measurement. The underlying message could well be true, that for some people in the UK indoor pollution for particulates may be higher than outdoors, but the data here doesn’t prove it to be so for the population as a whole.”


The data from the National Air Quality Testing Services was published at 00:01 UK time on Wednesday 14th May.


Declared interests 

Prof John Gulliver: No conflicts of interest. I received funding from MRC and European Union Framework programme for air pollution research.

Prof Alastair Lewis: Prof Lewis received funding from NERC, UKRI, Defra and BEIS for atmospheric chemistry and air pollution research, and from DSTL, Syft Technologies and Givaudan UK plc for analytical/instrumentation research. 

None others received

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