Earlier studies of humidity and the flu had focused on relative humidity and found no link, said lead author After reanalyzing these past studies, which also contained data on absolute humidity, Shaman's team found a much more powerful connection. "Absolute humidity conditions explain most of these changes," Shaman said. The researchers do not know exactly what it is about low absolute humidity that the flu virus likes. But they suggest that absolute humidity levels be raised in buildings such as hospitals and medical clinics where the disease most often spreads. "This gets us a big step closer to one type of mechanism" for how the flu spreads "One really key question is how much influenza is transmitted in tropical locations"—places with high absolute humidity year-round—"and how this compares to temperate parts of the world," he said.