Londoners celebrating Halloween were treated to a thick fog last week, but alongside the capital’s famous pea-soup the city was experiencing a series of large pollution peaks.

Pollutants transported from northern Europe may have added to emission levels in the capital. Events like this serve to highlight the growing need for international cooperation to tackle cross-border air pollution.

London’s air quality was deemed “Very High” in the early hours of Sunday morning. This means everyone would have been feeling the effects after roughly an hour’s exposure. High risk individuals would have immediately known something was amiss.

The peaks were due to high levels of particulate matter (both PM10 and PM2.5). These are small, solid particles that are generally more prevalent in the winter months. As you can see below, it was quite a spike!

At their most extreme, London’s levels of the more dangerous PM2.5 pollutant were higher than in some of the world’s most polluted cities: Shanghai, Mexico City… even nearing those of Beijing!

On an average day, London pumps out more than its fair share of harmful emissions, from everyday sources such as traffic and energy use. However, last weekend’s peaks were exacerbated by several reinforcing factors.

The situation could have worsened due to weather conditions not conducive to dispersing particulate matter, or city-wide fireworks displays to mark Diwali could have created extra emissions.

There’s a third possibility. We mapped the movement of the relevant pollutants back in time over three days, discovering that some of the problem may have blown in across the Channel.

We found similar peaks in Calais and Lille, with the same strength and frequency as those recorded in London. We should note that our model does not pinpoint the origin of the pollution episode, only its trajectory over a given time.

Pollution mapping is a difficult process, and this kind of modeling can only tell us so much. But it’s clear that reducing emissions in one city alone is not enough to prevent a negative impact.

External factors could exacerbate health risks even if emissions are usually low. It’s vital that countries work closely together to tackle atmospheric pollution. Don’t forget, you can always start taking active steps towards reducing your exposure.