Air pollution at national parks may be costing more than $4 billion in tourist dollars, according to a study commissioned by three environmental groups. Pollution in national parks in the East like the Great Smoky Mountains and Shenandoah has caused summer visibility to fall to 12 miles from 80 to 90 miles, while visibility in some Western national parks has dropped from 140 miles to 35 miles. Visitors to national parks rate visibility as one of the most important parts of their experience, the report says. It cautions that its findings relating to lost tourist dollars are “very rough estimates” because of the difficulty of quantifying what visibility is worth to the public. The U.S. EPA later this year is expected to issue new pollution restrictions to improve visibility at national parks by 15 percent each decade through 2060.