Particles of the allergen -- in this particular case, peanuts -- are small enough to actually float in the air, ... If they land on someone who is allergic, land in their eyes or in their nose, they can cause them an actual allergic reaction.
It's not all gloomy. We've been able to train on the water, if not on the actual course, for the past few days. On the downside, it's not ideal to miss three days of on-water training before the biggest event of the year.