Exactly ＿＿（31）＿＿ thunderstorms trigger or exacerbate asthma in this way is still not fully understood. The leading theory is that cold air downdrafts that occur during thunderstorm weather systems generate strong cross winds that blow at ground level, whipping up pollen grains and fungal spores from grass and plants. These then get carried high into the storm system by updrafts, ＿＿（32）＿＿ moisture in the clouds causes them to swell and ＿＿（33）＿＿ into smaller fragments, massively increasing the number of allergen particles in the air. The strong electric field that develops during thunderstorms may also enhance the rupture of the pollen. This smaller particle size makes ＿＿（34）＿＿ easier for the pollen fragments to get into the airways as they are carried back to ground-level by cold down-draft winds. Pollen levels seem to spike during the first 20-30 minutes of a thunderstorm, according to studies on the phenomenon. Younger people, particularly those with hay fever, ＿＿（35）＿＿ by this seasonal respiratory phenomenon than older adults.