Air pollution from two coal-burning power plants in Massachusetts can be linked to more than 43,000 asthma attacks and an estimated 159 premature deaths each year, according to a study released yesterday by the Harvard School of Public Health. Locals and enviros say the study gives Massachusetts Gov. Paul Cellucci (R) the evidence he needs to crack down on the dirty, antiquated plants. Under a loophole in the federal Clean Air Act, the plants are exempted from meeting current air quality standards. If they were required to conform to the higher standards, the number of deaths and asthma attacks could be reduced by 80 percent, the study suggested.