ALLERGY TO POLLENS: WHERE ARE THEY FOUND?
Pollens can be carried very great distances by winds in the atmosphere. Pollen grains have been found in the air in mid-Atlantic Ocean, and have been shown to be blown up to 300 kilometres (nearly 90 miles) a day. The average maximum distance travelled is thought to be closer to 50 kilometres (about 30 miles), however, and the majority of windbome pollens fall within a few hundred metres (yards) of their source. Most pollens from insect-pollinated plants will, as described above, be found in the immediate vicinity of their plant source.
In fine, clear weather in summer, grass pollens are affected by a marked daily cycle. Most grasses discharge pollen in the early morning, and these rise in convection currents and are carried up into the atmosphere with reactions being noticeable from 7.00 a.m. to about 8.00-8.30 a.m. Some species of grass discharge pollens a second time in the early afternoon. The pollens continue to rise and collect high in the sky if the weather stays fine, and then start to fall again in early evening as the warm air descends.
The worst time for many people allergic to grass pollen is this evening period as the pollens come down. The peak occurs between 5.00 and 7.00 p.m. in country areas, or places near parks and grassland. In cities, the same evening peak occurs, but usually one to three hours later, between 6.00 and 10.00 p.m. On calm nights, there is sometimes a second peak after midnight.
Pollen counts measure the concentration of pollen grains in the atmosphere, in terms of grams per cubic metre. A count of 50 is low, while a count of 100 to 200 is high. Counts of 800 grams per cubic metre have exceptionally been recorded. The concentration of pollen in the atmosphere increases as the season goes on, reaching a peak in early July in most parts of the UK. Rain washes pollen out of the atmosphere, and a heavy shower during the pollen season often reduces the pollen count significantly.
Tree pollens are known to exhibit the same daily cycle, and the same concentration of pollen count over time as grass pollens; but weather conditions in February to May are rarely conducive, and the season for each tree pollen is much shorter than that of grass.
Chemical pollution, such as vehicle exhausts in cities, can be an exacerbating factor for people with pollen allergy, many of whom find that air pollution holds down pollens in the low atmosphere and makes their reactions worse. In addition, if you are chemically sensitive, chemical molecules adhere to the surface of the larger pollen molecules and it is believed that chemicals are thus absorbed in greater concentrations during the pollen season.
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