← home

15 July 2021

London air

or, I can’t hack black snot

I really like living in London, but I don’t like the pollution. I’m no stranger to polluted air – I grew up in a big city where cars are the only way to get around – but there are some differences here. It’s denser, there’s more smoking in public areas, and more vehicles burn diesel, whose exhaust is harsh and chalky compared to the sticky sweetness of gasoline’s. Even so, I likely would have just allowed my body to adapt if not for one jarring feature of the London airscape.

Sometimes, home after a day out, I would blow my nose to find my snot black and sooty. I wanted to figure out what was happening, if for no other reason than to rule out a medical issue. Eventually, I realized that the Tube, which I thought was the most convenient and environmentally friendly way to get around the city, was the culprit. The source of my sable nasal discharge was the horribly polluted air in its underground lines. Most Tube lines are badly ventilated and full of, among other things, a lot of small particles emitted by the trains’ brakes and tracks. On deep lines, like Central and Victoria, these particles cake surfaces that aren’t touched often and make the air so dusty that you can see it move under the light. They’re also what turn your snot black.

With Tube air quality on my mind, I decided it was worth thinking up ways to mitigate the effects of bad air in London and cities more generally. I’m sure they’re nothing new to a seasoned Londoner, but I hope they may help other newcomers like me who can’t hack black snot.

* * *

Below I’ve listed limits suggested by the WHO1 for common outdoor air pollutants that pose health risks. They prescribe the maximum average concentration of a pollutant that should be allowed over a given period of time.

Pollutant Averaging Time Concentration (ug/m3)
Nitrogen Dioxide (NO2) 1 year
1 hour
Sulfur Dioxide (SO2) 24 hours
10 minutes
Ozone (O3) 8 hours 100
PM10 1 year
24 hours
PM2.5 1 year
24 hours

PM10 and PM2.5 are the most common proxy indicators for overall air pollution. They refer to all the different kinds of particulate matter – solid and liquid, organic and inorganic – suspended in the air that are small enough to enter and damage our bodies. PM10 particles are around 10 microns in diameter and can lodge deep inside the lungs, while PM2.5 particles are around 2.5 microns in diameter and can penetrate the blood stream.

Figuring out the actual air quality in London isn’t as straightforward as it seems. There is a network of sensors around the city, and Imperial College runs a website that allows you to view data from them.2 This website and other sources that monitor real-time air quality usually give users a pollution report using a single word like “low” or a single calculated summary value.

The problem with these reports is that they fail to capture the nuanced and localized nature of air pollution. For one thing, they don’t give a sense of what they’re measuring against. By default, the Imperial website uses the UK Government’s Air Quality Strategy Objectives, whose limits are less strict than the WHO’s. The sensor network is also low-resolution, and not all sensors record the same types of pollutant data. Extrapolating from what little information there is to present a single, easily-consumable pollution level for any given area just doesn’t give a clear picture of what’s going on. If you’re curious about air quality, I recommend looking at the actual pollutant levels reported by individual sensors and comparing those to the WHO guidelines.

In 2020, the London Mayor’s office published a report3 that compares levels of NO2, PM10, and PM2.5 recorded by London sensors in the years 2016 to 2019 against the WHO recommendations. Air quality in London is improving over time, likely due in large part to policy changes. From 2016 to 2019, the average annual pollutant levels across all London sensors decreased by 21% to 39 ug/m3 for NO2, 11% to 20 ug/m3 for PM10, and 9% to 11.6 ug/m3 for PM2.5. All of these values are hovering around the WHO limits, but there’s still a lot of room for improvement. The percentage of sensors reporting levels above WHO recommendations in 2019 was 42% for NO2, 35% for PM10, and 88% for PM2.5 (there were relatively few PM2.5 sensors and some issues with data capture).

Though these numbers might seem grim, it’s important to note that almost all of the sensors that reported levels over the WHO limits are on the sides of roads or located in other areas that are known to be polluted. Based on the report’s results, the highly localized nature of air pollution, and informal checking of data from sensors around the city, I think it’s likely that the air quality in uncongested residential areas is decent – not horrible, but not great either – even in central London.

When we move around the city, we often spend time in areas of concentrated pollution, like busy streets and tube lines. There’s no question that air in these areas is unhealthy. The sensor on the Strand reported 74 ug/m3 of NO2 in 2019, which is almost double the WHO limit. A 2019 Financial Times investigation4 found that PM2.5 concentrations in all of the Tube lines they examined were over the WHO limit, with parts of the Central line exceeding it over eight times, and they claim their readings are substantially lower than those reported in earlier scientific studies.

One of the best things we can do to reduce our exposure to bad air is change our modes of travel. Depending on what works for your journey, it’s best to go by foot, bicycle, bus, then Tube, in that order. Private cars are similar to buses from a personal air quality perspective, but worse for the air quality of the city as a whole.

If you’re walking, try to take side streets instead of major ones. When you inevitably do have to pass near traffic, walk as far from the road as possible. A few feet makes a huge difference in the amount of exhaust you breathe in. Sometimes it’s a game of dodging smokers on one edge of the sidewalk and idling cars on the other.

Cycling is similar to walking: it’s best to take quiet roads or dedicated bicycle paths where you won’t end up stuck behind an exhaust pipe at a red light.

In terms of public transit, if your schedule can handle it and the route isn’t too congested, then it’s much nicer to take the bus than the Tube. Of course, sometimes the Tube is the only reasonable way to get where you’re going. For those journeys, I recommend wearing a pollution mask while underground. Not necessarily a mask certified for industrial work, but one that makes a tight seal and has filters specifically for city pollutants. I use a mask designed for urban cycling. It’s not a burden, it doesn’t look too strange, and it keeps my snot a healthy color, even after long rides.

I also recommend bringing some kind of glasses when you move around the city, no matter the mode of transportation. They keep a quickly passing person, bicycle, car, or train from getting something unpleasant in your eyes.

At home, opening windows for ventilation is good for air quality as long as the air outside isn’t too polluted. If you live on a major street and there isn’t much distance between your home and the road, be careful not to open street-facing windows during periods of high traffic. An air purifier will also improve your indoor air quality. Look for one with a good, easily replaceable filter and a high air change rate.

  1. See the 2005 Air quality guidelines and a fact sheet on outdoor air pollution for more information on WHO limits. 

  2. londonair.org.uk 

  3. Air pollution monitoring data in London: 2016 to 2020 

  4. “London Underground: the dirtiest place in the city”