Oak Processionary is a moth whose caterpillars are often found in oak trees (also in some other species), and unfortunately several trees around Ealing, including the Hanwell Meadows and surroundings, are affected.
The caterpillars have poisonous hairs which can cause severe allergic reaction and trigger asthma attacks. Because the hairs can become airborne, you should stay well clear of any infected trees, especially in a downwind direction. Read more about Oak Processionary moth (OPM) on Wikipedia.
Please take care not to touch affected trees and do not disturb the nests.
Ealing Council has issued a renewed warning. See their June 2015 article for more details, identification help and health advise. Sightings can be reported via the Forestry Commission’s website at www.forestry.gov.uk/opm, by email to email@example.com or call the Forestry Commission on 0300 067 4442.