The Great Smog of London: Air pollution pt. 1

Tiny Matters

On December 5, 1952, the city of London was engulfed in a lethal black haze that killed thousands of people. This event, now known as the Great Smog, altered how London and much of the world viewed air pollution, and led to environmental policies that have no doubt saved many lives since then.

Transcript of this Episode

Deboki Chakravarti: For Londoners, December 5, 1952, seemed like it would just be a pretty typical winter day. But then things took a turn and for the next five days, London was engulfed in a lethal black haze that ultimately killed thousands of people.

Sam Jones: This event, now known as the Great London Smog of 1952, altered not just how London saw and dealt with air pollution. A lot of other countries were also thrown for a loop and this ultimately helped push forward environmental policies that have no doubt saved many lives since then.

Welcome to Tiny Matters, a science podcast about things small in size but big in impact. I’m your host, Sam Jones, and I’m joined by my co-host Deboki Chakravarti. Before we really kick things off, we want to thank listener Andrew for suggesting this topic because the stuff in our air is definitely one of those big things made up of a whole lot of tiny things and it effects everyone everywhere every day, which is why today’s episode is one part of a two-parter about air pollution.

Deboki: Today on the show we’ll be talking about where air pollution comes from, what it is, what that air quality reading on your phone is actually telling you, some of the successes that places have had clearing up air pollution, and the events that made that cleanup non-negotiable.

Let’s get back to the Great London Smog of 1952. Where did it come from?

Sam: It all started with the weather. Yeah, the start of the day on December 5 seemed pretty normal, but the days before had been quite cold, so Londoners had been burning more coal than usual. And that coal was not top of the line stuff. Higher quality coals were hard to come by, mainly because they were being exported to help pay off debts from World War II. This lower grade coal had a lot of sulfur in it, which meant that as it burned, a lot of sulfur dioxide was released in the smoke. And we’ll tell you why that’s important in just a second.

Deboki: So you have cold weather and people burning low-quality coal. And then on December 4th, an anticyclone arrived. Anticyclones are a type of weather system that do something called a temperature inversion, where a blanket of warm air traps the cooler air below. This is the opposite of what’s normal where air temperature typically decreases as you go up in elevation. So as smoke billowed out of chimneys and met the cooler air it stayed trapped beneath the warmer air above it, blocking out sunlight and cooling things off even more.

Sam: And, in response, people in London burned more coal to keep warm. And all of those pollutants were trapped in that cooler air, creating smog.

Smog is a mixture of fog and smoke and, like we mentioned, that smoke had a whole lot of sulfur dioxide. Scientists have hypothesized that some of that sulfur dioxide converted to sulfuric acid which is…an acid, so it’s corrosive which means it can irritate your eyes, your skin, and respiratory system. But sulfur dioxide itself is also really bad for your respiratory system and is known to cause breathing problems.

Deboki: And on top of all this, London had made some other recent changes that made the problem even worse.

Barbara Polivka: Shortly before the event, the city of London converted its electric trams to diesel buses. So we've got all of these things occurring at once. And for about five days, the smog was horrendous in the city of London. The historical descriptions of what was going on were just, to me, unimaginable, we just can't think about literally people walking off into the Thames River because they couldn't see where the dock ended and drowning.

Deboki: That’s Barbara Polivka, the associate dean for research at the school of nursing at the University of Kansas.

Barbara: One of the ways they knew that people were dying at a very exorbitant rate was the morticians ran out of coffins. They estimate about 4,000 people died in those five days, around those five days, and then another 12,000 later.

Deboki: London, and the rest of the world, was shaken. Something needed to change.

Barbara: Within about four years after the smog they did pass the Clean Air Act of 1956 in the United Kingdom that began cleaning up some of those issues. So it introduced things like smokeless fuels and looked at smoke pollution and how to reduce the sulfur dioxide. So yeah, that precipitated a large movement in the UK.

Sam: And in the United States, there were also things that happened around that same time that were not as deadly but still very concerning. In one case, a steel mill in Pennsylvania that consistently spewed sulfur dioxide, zinc, lead and cadmium into the air turned deadly when, in 1948, a weather event similar to the one that happened in London came to town, killing 20 people. And places like Los Angeles were very well known for their air pollution and smog. So in 1963, the US passed the Clean Air Act, which was—and continues to be—very successful at reducing air pollution. We’ll talk about a specific example of how much of a difference it has made later in the episode, but first, let’s get back to the Great Smog and what interested Barbara about it.

A lot of Barbara’s research is focused on environmental factors that cause asthma. And a few years back, she came across a study that explored the long-term impact of the Great Smog.

Barbara: So these folks did this really interesting retrospective study, where they were able to compare those folks that were in utero during the London smog to children that were in utero in the surrounding areas. As well as those that were in utero pre-London Smog and post, and they found those folks that were in utero had an increased chance of developing asthma as they became children and adults.

A lot of my research deals with indoor residential exposures to environmental triggers for individuals, primarily older adults, but adults with asthma. We collected information on their exposure to indoor volatile organic compounds.

Volatile organic compounds—or VOCs—are essentially chemicals that make it into the air and float around and that you can breathe in. Not all VOCs are bad for you. For example, you smell buttery vanilla coming off a cupcake because of certain VOCs that make it to your nose. Barbara looks at the bad-for-you VOCs.

Barbara: We specifically looked at 84 different volatile organic compounds. And we did this for approximately 190 older adults in the Louisville Kentucky area. And that area is interesting because, according to the American Lung Association, asthma rates are highest there.

Deboki: What Barbara and her colleagues found was that there were much higher concentrations of VOCs indoors than outdoors. And, from the batch of ones specific to the indoors, they zeroed in on a few dozen—things like chloroform or compounds known to come from cleaning products that have been linked to different respiratory conditions and asthma.

I think one of the other things that's come out of our research is how important it is for individuals to understand the indoor types of environmental exposures that they have. One of the things we've done in our research is give information back to everyone who participated about the quality of their indoor environment and where some of these volatile organic compounds might be coming from. And we found that just by giving that information back to them, later they've told us that they've made adjustments to their indoor environment.

We spend a lot of time in our home and have spent even more time in the last few years. And I think paying attention to the indoor environment is critical.

Deboki: At this point we’ve spent some time talking about the actual pollutants in the air—namely the sulfur dioxide released during the London Smog.

But what about other pollutants and sources of pollution? We talked with Chemist Luisa Molina who established the non-profit Molina Center for Energy and the Environment and is also affiliated with MIT. Luisa studies a lot of things related to chemistry and the atmosphere, but we spoke with her about air pollution in cities, specifically megacities which are cities with a population over 10 million people. She’s interested in working with those megacities to assess air quality and ultimately impact the environmental decision making process so that regional air quality and human health improve.

We started off by asking her, when she visits a city, are there things she immediately notices that would indicate there’s some kind of problem?

Luisa: The most important indicator is your health. You have the irritation of your eyes, the nose, the throat, and you have difficulty breathing. But then the other indicators, the haze, you have this reduction in visibility, you cannot see well. And the other thing that is a good indicator is the trees and plants have damage. These are visual signs, but then if you wanted to do it scientifically, of course, then you use air quality monitors.

Sam: So let’s talk about the pollutants people are most concerned with monitoring.

Standards and measurements vary around the world, in some places more than others. Because we’re in the US we’re going to focus on the US air quality index calculation, or AQI. In all likelihood you’ve seen an air quality number on the weather app on your phone. I am going to open mine right now and…. I am being told that the AQI today is…good. The score is 43. But what the heck does that mean?

The AQI is based on five pollutants that are regulated under the Clean Air Act. We have ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

Deboki: So let’s go through each of those five pollutants to talk about what they are and why they’re being regulated.

First, we have ground-level ozone. This a gas that forms just above the earth's surface and it’s created when nitrogen oxides and VOCs that come from things like car emissions react in the presence of sunlight. Unlike stratospheric ozone, which forms in our upper atmosphere and that we need to protect us from the sun's harmful ultraviolet rays, ground-level ozone is not good for you. Remember the LA smog we mentioned earlier? That was mostly ground level ozone.

Then we’ve got particulate matter. These are little particles floating in the air—could be things like dust around your house or burning fossil fuels. You’ll often hear them described as PM2.5 or PM10. The number has to do with how big the particles are and, although it may sound kinda counterintuitive, the smaller particles—PM2.5 —are more dangerous because they’re tiny enough to get deep into your lungs or even your blood.

Sam: Next up we have carbon monoxide. Breathing air with a high concentration of carbon monoxide actually makes it so that you can’t transport as much oxygen in your blood to organs like your heart or your brain. That’s because carbon monoxide takes oxygen’s place. Hopefully you have a carbon monoxide detector in your home. If you do not, please go get one as soon as you finish this episode.

And then the final two are sulfur dioxide and nitrogen dioxide. Sulfur dioxide we’ve already talked about quite a bit with the Great Smog of London. The important thing to know about nitrogen dioxide is that high amounts of it can cause chronic lung disease.

Deboki: So those are the 5 things and after hearing that I bet your’re glad they’re regulated. The EPA has established standards—what’s acceptable in terms of exposure for each of these pollutants. The scale is set up so that 0-50 is considered good air quality where pollution poses little or no risk, 51-100 is considered air quality with “moderate” concern, but still fine, 101-150 is unhealthy for sensitive groups, 151-200 is unhealthy, 201-300 is very unhealthy, and 301 and above is considered hazardous.

Sam: But Luisa told us that the standards are always changing as scientists learn more about those pollutants and what amount is acceptable for someone to be exposed to.

More research, more data—it’s a good thing. Because what it does is inform countries of how they should update their standards, helping protect human health and making the chance of another event like the Great London Smog unlikely.

Deboki: And we should also mention that geography is super important. In Delhi, India, for example you do have a lot of industrial activities and things like crop burning that shoot tons of pollutants into the air, but Delhi’s also land locked, so there isn’t an ocean breeze to transport pollutants so they stick around. In Delhi the PM2.5 is off the charts—something like 10 times what’s recommended by the WHO.
Sam: And even though we focused on 5 very important pollutants that influence the AQI you see on your phone, the EPA also regulates things like lead. Lead used to be in gasoline, for example, and is super dangerous because it can not only impair liver and kidney function, but it’s well-known to cause neurological damage in kids.

So, who or what are the biggest pollution perpetrators? Luisa told us that the sources of air pollution are typically split into 4 main categories. Three of those categories are anthropogenic—meaning they’re related to human activity—and the 4th category is natural.

The anthropogenic sources include: Mobile, stationary and area sources. So mobile—things like cars, buses, motorcycles, but also airplanes and boats and agriculture equipment. Stationary, also sometimes called point sources—you have things like factories, power plants, and refineries. Area sources include a huge range of things, so you have wastewater treatment plants, solid waste facilities, mining operations but also residential heating and cooling. And then the natural sources which are not always things I always think about when I think about air pollution, include thigns like windblown dust, lightning, volcanoes, and of course forest and grassland fires.

Luisa: In many cities and urban areas around the world, the emission from the transportation sector is the major source of the pollution.

Deboki: Think about all of those photos you saw when much of the world shut down at the start of the pandemic. Places where the air is typically thick with smog actually had views beyond the city, beyond the street across from you. Why that clearing? No cars and other fossil fuel combusting vehicles on the road.

Something similar happened in Los Angeles decades ago. Not because of a pandemic of course, but because of an invention called the catalytic converter.

Sam: There used to be no regulation on what was coming out of your car’s tailpipe. But then in 1970 you have an amendment to the Clean Air Act and the creation of the EPA who could actually enforce the Act. And they were like… we need to clean up this air. Especially LA because that smog was bad. When the catalytic converter came around shortly after, it cut down on most toxic car emissions by 99%. And according to the EPA, catalytic converters are one of the greatest environmental inventions of all time.

So let’s briefly talk about why they’re so great. Your engine burns gasoline to produce the energy that allows you to drive your car. Gasoline itself is mostly a bunch of different hydrocarbons that would, in a perfect world, react with oxygen to then produce water vapor and carbon dioxide which and those would just be released your car’s tailpipe.

But this is not a perfect world and incomplete combustion of those hydrocarbons will leave toxic gases like carbon monoxide and nitrogen oxides.

So, catalytic converters use a combination of metals to make a bunch of reactions happen in your engine so that your car can mostly release carbon dioxide and water vapor. Carbon dioxide is a greenhouse gas so this not 100% a “we fixed everything let’s pat ourselves not the back” situation, but the catalytic converter was still an incredible invention that has done wonders for our air and our health and the city of LA.

Sam: And, like LA, there are a lot of cities around the world that have also cleaned up their air.

But still, according to the American Lung Association, unhealthy air and smog continues to be a major issue around the globe, particularly in India and China and, in the United States, nearly 40% of Americans are breathing unhealthy air.

So how do we fix this? Luisa says by approaching it like it’s a global issue.

Luisa: What we really need is an integrated strategy. You need everybody work together. You need scientists, you technical experts and scientific experts, you need political science, you need social scientists, you need urban planners and the economists. All of these should work together with the decision makers, the policy makers. You need to work together with them from the very beginning so that they buy in. You need political will.

I work with so many emerging megacities. And it is really important for developed countries, developed cities to support them. That's absolutely essential.

Deboki: In this episode we’ve laid the foundation for what air pollution is, where it comes from, what that air quality score on your phone actually means, and we’ve talked about some of the strategies for cleaning it up.

In the next episode, we’re going to talk about an event that released a massive amount of air pollutants in a matter of not years, not months, not even days—it was all just in a matter of seconds.

Anna Nolan: I do recall walking to work that morning. It was a beautiful day. I came to work and we were doing our usual pulmonary fellow things like we were setting up for bronchoscopies and all sorts of things. And then we heard that a plane had hit the trade center. And then people were trying to rationalize, oh, maybe it was just an accident or maybe it was all sorts of other things that could have happened. And then we realized that the traffic had stopped on the highway, which we could see outside the window of the bronchoscopy suite.

And then we noticed that just at the point where the cars were starting at the extreme distance of my view that there were no cars, they were just ambulances. We finished up what we were doing and we came outta the room and we heard that another plane had hit the other tower. From there it was just, uh, the day kind of progressed and we're waiting for patients and to see what was gonna happen. And it just got worse and worse.

Sam: That’s environmental medicine researcher Anna Nolan, who was in New York City on the September 11th attacks. In the next episode of Tiny Matters, we’ll hear about her work studying the lungs of firefighters who were at ground zero. And we’ll also hear from a researcher studying that day’s link to cancer.  

Deboki: So before we sign off, it is time for our Tiny Science. Tiny Show and Tell. Our Tiny Show and Tell. I remember the name. So I will start today, and I'm going to start by highlighting this article that I really love. I just think it's delightful. It's from the New York Times. The headline is “The Search for a Model Octopus That Won't Die After Laying Its Eggs.” And the author of the article is Elizabeth Preston. And yeah, it's about the quest for a model octopus. And really just like, how do we turn cephalopods, which also includes other animals, like squids and stuff, how do we turn them into a model organism? And I'm just always fascinated by model organisms in general, because... I mean, it's just neat. It's interesting—

Sam: Yeah.

Deboki: ... how we figure out that these animals can, A, tell us things that are interesting scientifically. And also, how can we work with them? Because there's just so many practical considerations. In this case, with octopi, or octopuses, not octopi. The issue is just how do we get them to not die after they reproduce? This is a major challenge, because apparently females will reproduce and then they'll die. And that's just not great science-wise.

Sam: Right.

Deboki: So researchers were able to raise, I think, two to three generations of this one octopus. It's very cute, very small. And it's got these stripes on it. It's better known as the lesser Pacific striped octopus. And they're small, which makes them a little bit easier to maintain. And they're also one of the few octopus species that people have found where females are able to reproduce several times, which is why scientists are excited about them and their possibility as a potential model organism.

And so, there's a lot of great details in this article. If I just had to hear and listed all the delightful details about how males will vibrate their arm tips to get ready for mating and stuff, we would be here all day. So I will leave it to you all to read the article to learn more about why these octopuses are so adorable, but also practical for science. But yeah, that's my show and tell.

That's awesome. That's really cool. I'm excited to read it. So my show and tell, I'll just dive right into it. So over the last decade or so, there have been a ton of studies that have been published saying that nature is great for mental well-being. So in terms of things like happiness, and anxiety and depression. But recently, some researchers at the University of Vermont actually analyzed almost 200 peer-reviewed studies that were published between 2010 and 2020. And they found that the participants in the study were overwhelmingly white. And that over 95% of the studies occurred in high-income Western nations. You might be wondering, why do we care? Why does it matter? So taking such a narrow sample of humanity, it doesn't reflect the diversity of cultures and values across the globe. And that makes it difficult for the field to make these universal scientific claims.

There are parts of the world where there's extensive farming and ecological pressures that have seriously changed the landscape. And so, for people there, being out in nature might have a very different feel than for somebody say, like me. So the authors of the study see it as kind of a wake up call, hoping that it will lead to conversations about collaboration with diverse communities, how to include the use of culturally sensitive experiments and tools. And they also suggest different improved methods for training in cross-cultural research that would have a greater emphasis on equity and justice. So conversations about mental health, they've really come to the forefront during the pandemic. And I'm embarrassed to say that I hadn't considered that the inequity that exists in many of the studies that are underlying those conversations.

Yeah. And I think it's always useful to think about how these experiments are done. And when we're reading whatever headline about whatever experiment that someone's studying about how people respond to things. Thinking then about, well, how did they set up that experiment? How did they set up their study is always super useful.

Sam: Thanks for tuning in to this week’s episode of Tiny Matters, a production of the American Chemical Society. I’m your exec producer and host, and I’m joined by my co-host Deboki Chakravarti.

Deboki: This week’s script was written by Sam, edited by me, and fact-checked by Michelle Boucher. The Tiny Matters theme and episode sound design are by Michael Simonelli and our artwork was created by Derek Bressler.

Sam: A big thank you to Luisa Molina and Barbara Polivka for chatting with us for this episode.
Deboki: As always, if there are some tiny things that you think matter and that you’d love us to explore in an episode, shoot us an email at You can find me on twitter at okidoki_boki

Sam: And you can find me on twitter at samjscience. We’ll see you next time.


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