The particles big and small that make up Saharan smog



The UK news media has been buzzing with reports of air pollution alerts associated, at least in part, with the long-range transport of dust from the Sahara. Colleagues from Africa have asked why we in the UK are worried about the health effects of a relatively rare occurrence of this long-range dust all the way across Europe, when African countries experience dust storms of much higher intensity almost daily at some times of year.

What the UK air quality alerts, issued by the Department for Environment, Food & Rural Affairs, don’t account for is the cause behind this large number of microscopic particles in the air. Instead, the alerts are based on the total weight of particles that occur in one cubic metre of air at standard temperature and pressure. But only more sophisticated, and less routine, measurements can distinguish between particles from different sources, such as local traffic and/or environmental problems.

So while official alerts have gone out, we have yet to get a breakdown of the chemical make-up of these particles and where they are from. When we know this we can then work out what each of the possible sources is contributing to the overall pollution load.

It is not just chemical composition that matters; size does too. The current air pollution problem appears, in its worst hotspots, to be made up of the combination of a) Saharan dust particles with b) particles made in the atmosphere from gaseous pollutants, along with c) particles emitted directly from very nearby sources, such as traffic.

Particles are measured on a scale called particulate matter, or PM. The weight-based measurements split results into only two classes: PM2.5, which groups together the smallest particles, and PM10, which totals up all the particles. Local traffic produces millions of nano-particles – the smallest particles we are interested in for this part of the story. Let’s imagine that instead of being really, really, small, these nano-particles are the size of footballs. On a relative scale, this would make the particles made in the air mostly about the size of a large car and the Saharan dust particles would as big as a small detached house. PM2.5 weighs the footballs and cars, while PM10 weighs everything: footballs, cars, houses and all.

When you think of it like this it is easier to imagine how different these particles are and how they might behave in the air as they travel through the human breathing tract.

Most of the medical statistics that look at the health effects of these microscopic particles are, like the alerts, “blind” to their source and chemical composition, but there are some studies that suggest that it is the traffic-based nano-particles that are worst for human health because they are small enough to move between the lungs.

The particles made in the air harbour strong acids within them and so can be very damaging to ecosystems, and are certainly not a welcome addition to the air we breathe. The Saharan dust particles, being big and easy to trap in our noses and throats, may be the most benign health-wise and environmentally-speaking are known to have important beneficial effects in providing nutrients to remote regions.

What we can take from this is that the air pollution event is a more complicated story along with the explanations for why long-range transport of Saharan dust has tipped south-eastern UK into an air quality alert when millions of Africans live with huge atmospheric loadings of Saharan dust almost every day. Temporary exposure is unlikely to affect otherwise healthy people but if people are worried, and especially if they are vulnerable to respiratory problems, they should certainly take heed.