Scientists think they have identified one key reason why ground-level ozone remains stubbornly high in Europe.

BBC: London's clue to stubborn ozone levels

Ms. Jackson late last year delayed a decision on whether to tighten the government's limits on ground-level ozone, a primary ingredient in smog.

WSJ: EPA Tangles With New Critic: Labor

But the traffic increase has slowed progress in reducing emissions further, and people in many areas still suffer from nitrogen-related air pollution, including small particulates that get sucked deep into the lungs, and ground-level ozone - a strongly irritant gas formed by the action of sunlight on reactive nitrogen.

BBC: Nitrogen pollution 'costs EU up to ?280bn a year'