Studies of the first SARS-CoV in 2003 suggest weather might be important for coronavirus spread. While this virus did not circulate long enough to establish any potential seasonal pattern, daily weather was associated with the number of cases. In Hong Kong, new cases were 18 times higher in lower temperatures — under 24.6°C, 76°F — than more elevated temperatures. The epidemic died out during a warm, dry July in 2003, but tight public health control measures were also in place. A recent review of the seasonality of respiratory infections describes how cold, dry winter weather makes us more susceptible to viruses in general. In these conditions, the mucous lining in our noses dries up, which in turn impairs the function of cilia, the tiny hairs that line the nasal passage. These beat less often, meaning they may fail to clear viruses from the nose. The review concludes that a relative humidity of 40–60% might be ideal for respiratory health.