If pollution affects health and the most affected respondents leave the sample, then our results will be biased.
Dr Penny Woods, chief executive of the British Lung Foundation, said: "We urgently need a new clean air act that restricts the most polluting vehicles from our urban areas and protects everyone's lung health – air pollution affects all of us".
And, while some air pollution affects people in low-income communities more, ozone is an equal opportunity polluter.
Air pollution affects economies and people's quality of life.
Air pollution affects not just the big cities.
"This pollution affects cloud formations, precipitation, storm intensity, and other factors and eventually impacts climate.
The quality of public discourse, like air pollution, affects the whole atmosphere.
The burden of air pollution affects those beyond a country's borders.
We're looking for stories on how air pollution affects our readers' lives, wherever they are in the world.
Theoretical uncertainties are how pollution affects cloud formation and the planet's overall climate "sensitivity", ie how much it responds to extra CO2 in the atmosphere.
"People who are suffering from asthma are sensitive, they might see an exacerbation of their condition, but pollution affects the whole population," he added.
