Pollution update

Last May, I did a bit of research on pollution levels in Putney High Street, Chiswick High Road and Castelnau in Barnes. Here's a link to that article.

At the time, I concluded that pollution levels hadn't got worse at any of the sites, and at Putney and (of course) Castelnau, it could be argued that they have improved. I decided to have another look at the three sites, to see if the there has been any change in this trend.

I chose the sites because there has been a lot of grumbling about the increase in traffic over Putney and Chiswick bridges since Hammersmith Bridge was closed to cars.

Putney High Street

The nearest monitoring site that I could find to Putney Bridge is in Putney High Street. Here is a graph showing the daily mean of four pollutants from 11 April 2018 (a year before the bridge closed) to today.

Pollution levels in Putney High Street. Source: King's College London

Pollution levels in Putney High Street. Source: King's College London (Click to zoom in)

As before, the level of pollution doesn't seem to have increased, and in fact seems to have fallen overall. There are a couple of strange peaks at the end of 2019 and the start of 2020, but I'd say the average values are lower than before the bridge closed.

The London Air Quality Network also allows you to download data, so I thought I'd look at the statistics for Putney High Street in more detail. Here are the measurements for two types of pollutant; I've looked at the annual mean, and the levels for each month, for 2018, 2019 and 2020.

  Ann. Jan Feb Mar
Jul Aug Sep Oct Nov Dec
2018 67 76.2 73.6 66.5 70.8 63.0 55.1 65.1 67.1 73.2 73.6 57.1 68.3
2019 69 86.5 78.0 76.5 58.4* 71.7 60.4 70.4 62.7 62.6 63.5 70.8 64.1
2020 60 62.0 59.3                    

Mean levels of NO2 (µg/m3) *Bridge closed April 10th 2019

  Ann. Jan Feb Mar
Apr May Jun Jul Aug Sep Oct Nov Dec
2018 25 23.2 27.7 29.8 27.2 31.6 24.0 25.6 21.0 22.3 21.5 24.0 20.2
2019 22 26.3 33.5 22.4 33.0* 20.6 18.8 18.3 17.9 18.9 16.6 17.4 18.2
2020 21 21.6 21.2                    

Mean levels of PM10 (µg/m3) *Bridge closed April 10th 2019

The table for NO2 shows a slight rise for 2019 (during which the bridge was closed to traffic). It's not a big rise, but nevertheless it could be argued that the situation was made worse by the closure of the bridge, and indeed there is a sharp increase in May 2019, the first whole month after the closure of the bridge. However, you'll see that in the three months before the bridge closed, pollution was worse in 2019 than 2018; and if you look at August through to December, all months apart from November show a decrease. So although pollution for the year was slightly higher, the levels do seem to have fallen after the bridge was closed to motor vehicles.

The table for PM10 shows higher levels in 2019 than 2018 before the bridge was closed, and then a clear decrease after the closure. The decreasing trend appears to be continuing into 2020 as well.


There doesn't seem to be a monitor particularly close to Chiswick Bridge, but here is a graph for the same period for Chiswick High Road:

Pollution levels in Chiswick High Road. Source: www.airqualityengland.co.uk

Pollution levels in Chiswick High Road. Source: www.airqualityengland.co.uk (Click to zoom in)

Again, I'd say we can see the same pattern as for Putney: pollution is no worse than before, and may have actually improved.


As in my previous article, here is the graph for Castelnau (the road that leads to Hammersmith Bridge, which would typically carry very long traffic jams in the mornings and evenings):

Pollution levels in Castelnau, Barnes. Source: King's College London

Pollution levels in Castelnau, Barnes. Source: King's College London (Click to zoom in)

It's no surprise to see that pollution levels in Castelnau have fallen dramatically since the daily traffic jams have disappeared. In fact, Castelnau is now an oasis of calm, and a pleasure for both pedestrians and people on bikes to travel along.

You can see the same two peaks towards the end of the graph, as in the graph for Putney. I can't think what may have caused them - if you think you might know, please get in touch.

So, if pollution hasn't actually got worse - and has even got a little better - how can it be that so many people who say it HAS got worse are wrong? Well, the human memory is a pretty fallible thing. Although we don't like to admit it, we really have trouble remembering things accurately.

If you're sitting in a car, stuck in traffic on Putney High Street, you'll most likely blame the delay on the closure of Hammersmith Bridge, and add that it must be making pollution worse. But you're forgetting that (a) traffic on Putney High Street has always been terrible, and (b) pollution on that road has always been incredibly high. (You're also forgetting that you are a part of the cause of the traffic - but that's a topic for another day.)

And, if we accept that pollution actually hasn't got worse, then why hasn't it? After all, you'd assume that people who were previously driving over Hammersmith Bridge would have switched to either Putney or Chiswick bridges. Maybe this is evidence that traffic evaporation has actually happened, and people who were previously driving are now using other means of transport to make their journeys.

Posted on 2nd March 2020

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