But she really shouldn’t have been, according to a new study from Finnish researchers. The researchers found that a single 30-minute sauna session increased the number of circulating white blood cells. The sauna bath in effect mobilized white blood cells into the bloodstream from tissues, where they are better able to patrol the body and respond to pathogens. By sending more of these cells into circulation, the body may improve immune surveillance and strengthen its ability to respond to possible threats.

But before you run out and buy that big sauna for your backyard, let’s recognize the limitations of this study. This was a study of a single sauna session, and, not surprisingly, the release of the white blood cells into the bloodstream did not last long.  It is thus unknown whether a regular course of sauna would have a more durable effect on immune function or long-term health.  But suffice it to say at this point that the research opens up the possibility that the white blood cell response is the source of the positive health results long associated with saunas.