A few years later, a new diagnosis began to spread around the country, based on a nebulous array of symptoms including sore throats and headache that seemed to be associated with indoor air. Epidemiologists called the illness “Sick Building Syndrome,” and looked for its source in large-scale heating and cooling ducts. Even today, the particulars of the condition—and the question of whether or not it really exists—have not been resolved. But there is some good evidence for the idea that climate-control systems can breed allergenic mold or other micro-organisms. For a study published in 2004, researchers in France checked the medical records of 920 middle-aged women, and found that the ones who worked in air-conditioned offices (about 15 percent of the total pool) were almost twice as likely to take sick days or make a visit to an ear-nose-throat doctor.

This will come as no surprise to those who already shun the air conditioner and worship in the cult of fresh air. Like the opponents of A/C from a hundred years ago, they blame the sealed environment for creating a miasma of illness and disease. Well, of course it’s unhealthy to keep the windows closed; you need a natural breeze to blow all those spores and germs away. But their old-fashioned plea invites a response that’s just as antique. Why should the air be any fresher in summer than winter (when so few would let it in)? And what about the dangers that “fresh air” might pose in cities where the breeze swirls with soot and dust? A 2009 study in the journal Epidemiology confirmed that air conditioning can help stave off the effects of particulate matter in the environment.