Weather conditions determine when 'hay fever plants' flower and how much pollen they release into the atmosphere. Every year, the progression of hay fever symptoms can differ. The daily pollen concentration in the atmosphere in the Netherlands is measured by the Leiden University Medical Center (LUMC) and Elkerliek Hospital in Helmond. As part of the Nature’s Calendar
observation programme, secondary school students and volunteers keep track of when various hay fever plants begin flowering. The students who conduct observations are also involved with the GLOBE Programme, an international education programme on the environmental sciences established in 1995 by Al Gore.