I have been made aware of a relatively new problem in the UK principally affecting oak trees. It is caused by the "oak processionary moth" which defoliates the trees - the problem with these moths are in the caterpillars whose hairs contain a toxin "Thaumetopoein" which when in contact with the skin causes irritation principally. In some the reactions are a lot worse resulting in anaphylaxis. Inhaling the hairs may also bring on breathing difficulties. Lots of info on Forestry Commission and HPA websites. Share with your gardens/outdoor teams.