I am just going to answer this question.
Yes, the WHO has already lowered what it thinks is the safe limit, which I think demonstrates how complicated the issue is. It would be wrong to set a number on the face of the Bill without being absolutely certain that it was the right one—as my hon. Friend understands. I have spent a great deal of time on this issue with academics and scientists, and I am happy to share with others if that is helpful. We must make sure we get this right before we set the target. To be clear, to achieve even the 10 micrograms per metre squared in our cities would require significant change in all our lives. It would likely introduce policies aimed at restricting traffic kilometres by as much as 50% or more, a total ban on solid fuel burning including wood, and significant changes to farming practices to reduce ammonia, which reacts in the air to form particulate matter.