Boston Globe: The sweltering temperatures have hardly begun, but unhealthy, smoggy air has permeated parts of New England 18 days already, and many more are in the forecast. It’s not that the air suddenly got dirtier. Rather, the federal government in March lowered the threshold for declaring the air unhealthy, based on research showing that smog is far more harmful at lower concentrations than previously believed. As a result, environmental officials say there will probably be substantially …