In recent years there have been many attempts to use cigarettes-per-day as a way of communicating the health impact of particulate matter (PM) pollution around the world, e.g. "PM2.5 levels were so high in Paris this week that every individual walking the streets on Tuesday inhaled the pollutant equivalent of 47 cigarettes."

In a paper published in 2015 the Mullers compared "air pollution on a particularly bad day in Beijing to smoking 1.5 cigarettes every hour." This builds on research published in 2011 by Arden Pope in which he analysed the pollution in Beijing and stated that levels were equivalent to smoking 0.53 cigarettes/day.

More recently there has been some discussion, if not controversy, over quite how accurate these comparisons are with more work being done to establish the conversion factors between PM2.5 pollution in µg/m3 and cigarettes per day.

It is a convenient and easy way of helping people to relate to the threat that PM exposes people to in the course of their normal day. The problem is that unless pains are taken to define terms very closely we are comparing apples with oranges.

Cigarettes cause damage in multiple ways, nicotine, particulate matter, carcinogenic material, hot ash/air. What exactly are we comparing to what when we say that the levels of atmospheric pollution today are equivalent to 1.27 cigarettes? How do you distinguish health effects or mortality rates due to cigarette PM inhalation from those due to cigarette nicotine exposure? Does PM cause damage in the same way as cigarettes? Are there circumstances in which PM is more like one aspect of cigarette pollution than another? Confusion arises easily.

What is helpful about the analogy is that it communicates clearly the nature of the problem: that it occurs by inhalation, and that most people are passive victims to the pollution produced by others, even if they are PM producers themselves. It is also useful to note that we understand it is especially the PM resulting from combustion events which cause the most devastating problems. The other major positive about the analogy is that it captures something of the scale of the problem: just as smoking is a public health issue so is PM pollution. Although on this last point I think cigarettes let us down! PM affects everyone in an area, smoking only affects those near to smokers. It is normally easy enough to move out of a smoker's smoke, it is many times more difficult to escape PM pollution when it is blanketing your home town or city or region.

While expressing the impact of PM in terms of cigarettes/day is relatable, it lacks precision, and any attempt to tie things down too closely is, in the humble opinion of this blogger, more confusing than it is worth. The objective with the comparison is surely to communicate a level of risk, but then we could just as well use something dissimilar as the reference point, like rock climbing, lithium batteries, or bacon sandwiches. Each one is dangerous to health in some way, each one is relatable, and given the known mortality rates for each activity a comparison could be made. I am, however, unaware of any research into the mortality rates of rock climbing while using a Note 7 and eating a bacon sandwich at the same time...

Perhaps the whole comparison with cigarettes thing is more useful for the contrasts it sets up than the similarities.

In contrast to cigarette smoke PM can be formed from very many different types of material, varying even in their physical state, some is solid, some is liquid.

In contrast to cigarette smoke PM is not addictive, but it is nevertheless a pervasive toxin that exerts its influence on us daily.

In contrast to cigarette smoke PM may be produced a very long way away from its victims, but the impact is devastating nevertheless.

In contrast to cigarette smoke PM is not very well understood yet, nor does it receive the attention it deserves in terms of research or control on its production and spread.