The fight against indoor pollution

Indoor pollution is responsible for as many deaths as outdoor pollution, but goes unnoticed
10 February 2023

Interview with 

Ally Lewis, University of York


Bad air sits at the top of the WHO's list of global health threats and it kills millions. But we're not talking just about the pollution in the street: the elephant in the room is pollution indoors, which - it turns out - kills just as many people as pollution exposure outside, yet gets only a fraction of the attention. England's Chief Medical Officer, Chris Whitty,  together with public health and atmospheric chemistry colleagues, wants to change that. They've laid out a plan for getting to get to grips with the unknowns around indoor air pollution. Ally Lewis is from the University of York…

Ally - Everybody pictures outdoors as the place that pollution exists. But actually around 3 to 4 million people per year die because of their exposure to indoor pollution. So it's a huge global health issue. Understandably, for a long time, perhaps 200 years, the focus of governments and industry was in cleaning up outdoor air. Outdoor air quality historically, in places like the UK, was really terrible. There have been huge improvements in outdoor air quality in the last 50 years and particularly so perhaps in the last 20. So although there's a long way still to go with outdoor air, we have to look about whether we can get some equal or perhaps even greater health benefits by cleaning up indoors. So the focus on indoor air now is partly because of the success we've had in cleaning up outdoor air.

Chris - You've written this opinion piece this week with a number of other authors, one of them, the chief medical officer, Chris Whitty. You've set out five areas that you think warrant particular focus. I'll just put those to you and perhaps you could comment on why you've chosen those particular areas. You've led with 'Understand what's harmful.'

Ally - If we are going to introduce interventions or introduce policy or introduce technical fixes, we want be sure that we're dealing with the most harmful components of the air that we breathe indoors. And this is still an area of uncertainty. Chemical emissions from burning the solvents that we use from even things like cooking, there's also a lot of biological material indoors, so spores and moulds and respiratory viruses that we exhale. So before we design our solution to the problem, it would be really good to understand which of those targets is the one that's likely to give us the most benefits.

Chris - And I suppose that leads on to your second point, which is that you want to model how pollutants form and how they accumulate.

Ally - Indoor air is different to outdoors. The biggest difference being that it sees a lot less sunlight and, in outdoor air, sunlight drives a lot of the chemistry that transforms pollutants. This is something that we have to be able to simulate properly because when pollution is released indoors, we've got to be able to follow it through its full life cycle. And this ability to predict what happens to pollution indoors is much less advanced than we have for outdoor pollution.

Chris - The issue you're grappling with though, and I think this is really what you point to in your third point, which is 'explore the effects of local variations.' No two buildings, at least in the UK, are the same. And they're all different ages, different designs, different structures, different furnishings, different people, different people densities. All these things mean there must be so many degrees of freedom. How can you do this meaningfully?

Ally - It's incredibly varied indoors and it's much harder to predict than air pollution in outdoor spaces. But we think that there are some prospects to make really good progress. So one area that we would like to see advances in is the estimation of emissions. So outdoors, we know very precisely, for example, how much emissions come from the back of the diesel car driving down the street. We'd like to get those same sorts of estimates for emissions that occur in indoor spaces, for example, from a wood burning stove or a gas cooker. The next step would be to understand a little bit more about the ventilation rates in homes. And then finally we'd like to know where the pollution ends up - what is its ultimate faith.

Chris - It's interesting you bring up the point about accumulation indoors because I've watched quite a few episodes of grand designs now (which I like very much by the way) people seem to be striving for building homes that are almost hermetically sealed. They pride themselves on having a poorly ventilated home to make them extremely energy efficient. But are we not in danger therefore of trading an energy cost for a health cost? Because basically when you live in an ancient property like I do and it costs a fortune, but you know it's well ventilated because there's more holes than walls. With these modern buildings, we're sort of trapping the air into a sort of chemical stew pot that we sit in the midst of, aren't we?

Ally - There is certainly a risk that as we improve energy efficiency in homes, for very good reasons, it reduces greenhouse gas emissions, it reduces costs but we do end up leading to indoor environments where air becomes more and more polluted. We accumulate more and more chemicals indoors and we have to address that. And there are technical fixes to this. It is possible to retain the heat that still allows fresh air to enter, but there is a certain amount of trade off here between energy efficiency and indoor air quality.

Chris - And you also propose that we need to understand the best ways to improve indoor air quality. What sorts of things should be at the top of the list?

Ally - In a place like the UK, if it's wet and windy, certainly the best way to improve air quality is simply to open the window a crack and improve the ventilation rate. But that might not be the best advice in the middle of summer, in the middle of heat waves when things like photochemical ozone begin to accumulate. So the advice that we provide is going to have to be dependent on the location. It may be different depending on the meteorological conditions. And finally it might actually be different depending on where your house or your school or your business is. You know, those in rural environments generally can use outdoor air to clean up while those that are right by busy roads might have to take different approaches.

Chris - And lastly, you say you want to bolster the evidence base, strengthen the science. What do you really want to know?

Ally - At the moment, it's quite difficult to provide advice to decision makers or to building owners over which actions they might take that would have the largest beneficial effect. And quite understandably, people are reluctant to take action, to invest money, to use up capital. It's trying to persuade people to live differently, giving them the confidence that they're being provided the best possible advice or they're tackling the sources that are going to lead to the largest health benefits if we make changes. So this is where we need to bring more scientific evidence into the debate to give confidence to people who may have to make decisions or provide us with the nudges that we need to improve indoor air.


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