Safe levels: the riddles of exposure to particulate matter

Enjoy this new report from ASH Scotland's Refresh project, funded by a Big Lottery grant. 'Refresh' is a near-acronym made up of the words 'Reducing Families' Exposure to Second-hand Smoke', and the report guides people working with expectant mothers and fathers to the best way to reduce secondary smoke exposure.

Interestingly the report doesn't mention third-hand smoke, but it does insist that there are 'NO safe levels of exposure to second-hand smoke'. As ever, there is a rider to this: the World Health Organisation has set a 'safe level' of PM2.5 (fine particulate matter, which ASH Scotland claims to be constituted chiefly of smoke). The document says:

• The World Health Organisation (WHO) recommends levels of indoor airborne fine particulate matter (PM2.5) should be below 25 micrograms per cubic metre of air (25 μg/m3) averaged over 24-hours. 

• The average PM2.5 level in a typical smoker’s home is four times higher than the WHO guidance limit.

• A typical car journey where one person smokes exposes non-smokers to levels of PM2.5 that are about three times higher than the guidance limit.

The document is poorly referenced: the first bullet point gives as its reference Bonn: The WHO European Centre for Environment and Health; 2010, but the other two don't give a reference. An intriguing omission, given the figures involved. Recently we were told that the levels of toxins in a car containing a smoker would expose a passenger to levels of toxins 23 times higher than in a smoky bar. This figure was quickly corrected to 11. 


Taken together surely we are being told that passengers are being exposed to 100 micrograms per cubic metre in a car, but for a smoky bar exposure is only 9 micrograms per cubic metre, well below the guidance limit.


All the figures are of course plucked from the air (where are the references?). It is impossible to define the level of fine particulate matter in 'a smoky car', 'a smoky bar' or the home of a 'typical smoker'. Too many variables are involved even to give a sensible average. It is stretching credibility to claim that exposure to smoke in cars is any greater than smoking in a bar, never mind eleven times greater. It is also hard to believe that somebody's home is smokier than bars used to be.

More on PM2.5 measurements here.

Here, find a resounding endorsement of ASH Scotland from the members of the Scottish Parliament.