21.   Also reducing pollution levels, Massachusetts and other states have carried out smog-reduction programs, such as fume-trapping hoods on gas pumps.

22.   As pollution levels dropped to half Wednesday, an apparent result of cooler air and fewer cars, traffic is to return to normal on Thursday.

23.   Austin has exceeded acceptable pollution levels three times this year, as measured by revised air quality standards the Environmental Protection Agency issued last month.

24.   Air pollution levels in Maricopa County exceed federal standards, threatening to trigger new regulations and the loss of federal funds.

25.   But activists argue that biking should be encouraged, not penalized, as the San Francisco area works to reduce air pollution levels.

26.   But any penalties that they might incur for not reducing their pollution levels would be meted out only after the decade had passed.

27.   But however hardy wild plants turn out to be, the researchers stress the importance of continuing to reduce pollution levels.

28.   But these models are used mainly in hindsight, to figure out why pollution levels were so high and what can be done about them in the future.

29.   Environmental officials say the pollution levels are generally not high enough to cause massive respiratory problems for the public.

30.   Gathering information, rounding up all the affected parties, negotiating contracts, monitoring pollution levels and so on are all costly.