Rotterdam has little woodland and a great deal of port, and for hay fever sufferers that is good news rather than bad. The major sources of tree pollen are not in the city: the Kralingse Bos and Het Park are the exceptions, with alder and birch along the water. The port area and the Maasvlakte produce almost nothing, because very little grows there that is pollinated by wind. What the city does receive almost always comes from somewhere else.
That makes wind direction the single most important factor here. Under the prevailing south-westerly, clean moist sea air flows in and concentrations stay low, markedly lower than inland. When the wind turns east or south-east the picture changes within a day: tree pollen arrives from Germany and Belgium, grass pollen from the pastures of Brabant and South Holland, and Rotterdam can end up higher than Utrecht. The grass season, from May into July, is the hardest part of the year for most residents; the verges along the ring road and the meadows towards Barendrecht and Ridderkerk are close enough to be felt.