Climate change can exert a range of effects on pollen, which might have consequences for pollen-allergic patients. New allergenic pollen types might appear in the Netherlands, like common ragweed and olive, which result in allergy patients developing allergies that scarcely occur in the Netherlands at present. Trees, such as birches and planes, might produce larger quantities of pollen, which could result in more severe symptoms. The pollen season might become longer thereby extending the period in which patients suffer from allergy symptoms. This extension of the pollen season could be due to a prolonged flowering period of certain species, e.g. grasses, or the appearance of new species that flower in late summer, e.g. common ragweed. Climate change could cause an increase in heavy thunderstorms on summer days in the grass pollen season, which are known to increase the chance of asthma exacerbations.