The BBC's Michelle Roberts, who it seems has reprinted a press release from the zealots at Aberdeen University without asking a single significant question about its balance, verity or probity, is probably happy in her ignorance should she live in one of the UK's congested cities. She's lapped-up the 'smoking in cars must be banned' message without even bothering to look at the basic flaws in the scientific reasoning.
First, she mentions the WHO guideline air quality level for PM2.5 of 25 μg/m3 as a 24-hour mean. Then she identifies Aberdeen's background level for PM2.5 at 7.4 μg/m3. Ok, fine so far. Then she notes that particulate levels in a car with a smoker on journeys lasting from 10 minutes to an hour reached up to 85 μg/m3. R i i g h t. So if a child spends an hour a day in a car with a chain-smoker in Aberdeen, Michelle, what will its 24 hour mean exposure level be? And will this exceed the WHO recommendation? (The answers are 10.6 μg/m3 and 'No', Michelle)
And astonishingly for the 'Health Editor' of BBC news online she appears wholly ignorant of background levels of PM2.5 in places other than clean and airy Aberdeen. In London, if you walk or cycle near Marylebone Road, the Blackwall Tunnel or Woolwich Flyover, you'll be exposed to 24 hour mean PM2.5 levels of up to 86.5 μg/m3, 69.6 μg/m3 and 38.3 μg/m3 . Not hourly peaks, 24 hour means.
So Michelle, the science really 'proves' that a child in North London in a non-smoking house is exposed to over 8x the particulates level of a child in Aberdeen who spends an hour a day in a car with a chain-smoker.
(London particulate levels from London Air Quality Network site maintained by KCL at http://www.londonair.org.uk/london/asp/advstatsvariousresults.asp?site1=MY7&site2=TH4&site3=GR8&site4=&stattype=rmax&xvalue=98&zunits=none&startdate=01-jan-2012&enddate=30-aug-2012&submit=View&period=dailymean&species=PM25)
To be read in context with my response in the comments (clicky to make big)