The tactic du jour for environmentalists trying to sell a skeptical public on tighter regulations is this: spin the thing as a job creator. Last week a Maryland-based environmental group said efforts to clean up the Chesapeake Bay would actually create 240,000 jobs over the next several years, mainly by employing people to upgrade sewage systems. In a recent report defending stricter mercury pollution limits on power plants, the Environmental Protection Agency said 8,000 more people would be needed to build and run the pollution control equipment than would be laid off as a result of older plants shutting down. Economists that aren’t aligned with either industry or activist groups say that, when it comes to creating or destroying jobs, environmental regulations come out somewhere near neutral — adding costs to industry but producing benefits in public health or other areas.