Jackson appears regularly on the list of the worst cities for spring allergies (Here's what to do if you're a runner and have the condition). It was No. 1 last year as well. The pollen is too thick there. Moderate temperatures, foliage and humidity have created perfect conditions (5 Tips for Running in Heat and Humidity). “Humidity from precipitation also causes growth of fungus and release of mold spores,” Dr. Borczuk says.