At ground level ozone is a dangerous pollutant, but at altitude it blocks the passage of harmful ultraviolet light.

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In the Dallas-Fort Worth area, as total gas production in 2009 increased by 94 percent from 2000 levels, the levels of ground level ozone fell by nearly 15 percent.

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

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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.

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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.

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There is no paleoclimatic comparison for what this might mean, because there has never before been a significant amount of ozone at ground level.

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