Pollen in the Indoor Environment
What pollen is
Sources of pollen
Health problems caused by pollen
Decreasing exposure to pollen
What Pollen Is
Pollen is a living, biological contaminant that can travel through the air. Pollens, like other biological contaminants, contribute to poor indoor air quality (IAQ) and may be a major cause of days lost from work and school due primarily to allergic reactions.
Pollen is plant material usually generated by outdoor plants. The types of pollen that cause most allergic reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not have showy flowers. These plants produce small, light, dry pollen granules that are custom-made for wind transport. Seasonally, pollen is released from these plants as part of the fertilization process. Since pollen is so light, it floats on currents of air. This means it can travel for hundreds of miles. Samples of ragweed pollen have been collected 400 miles out at sea and 2 miles high in the air. Some pollen has even been found in Antarctica having been blown there from hundreds of miles away!
In addition to the allergic reaction that pollen can cause, often pollen can collect pollutants on its surface during its many hours of travel on the airflow. These pollutants could be toxic themselves, or they could cause trouble by enhancing the allergenic properties of the pollen's surface proteins. Some researchers are beginning to think that the growing incidence of asthma may turn out to be linked to pollution-laden pollen.
Sources of Pollen
Pollen is generated by plants such as grasses and birch trees, which rely on wind rather than bees to spread their pollen. In the US, weeds are a particular problem. Ragweed, in particular, is probably the most noxious hay fever plant in the world. Its pollen contains a protein that irritates the most tolerant immune systems. In the Mediterranean area of Europe, olive trees cause the most problems. In Scandinavian countries, birch trees are the most common source of pollen.
Because pollen comes from outdoor plants, indoor levels of pollen are lower than outdoor levels. Pollen generally enters the house through open doors and windows, cracks, on shoes and clothing, or can be brought in by pets.
Health Problems Associated With Pollen
Pollen allergy, commonly called hay fever, is one of the most common chronic diseases in the United States. In the case of pollen allergy, the antibody is specific for each type of pollen: one type of antibody may be produced to react against oak pollen and another against ragweed pollen, for example. The higher the concentration of a particular pollen in the air, the greater the chance that some people will become sensitized to it.
In allergy-prone people, the body's immune system becomes sensitized by repeated, massive exposure to the proteins on the surface of pollen. As these foreign proteins make contact with the moist, tissue inside a human nose, the immune system releases histamine. The histamine can then trigger the typical symptoms of hay fever, which are:
Running, sniffling nose
Itchy, watery eyes
Because these problems are caused by a release of histamine in the body, these symptoms may be treated with medication called antihistamines.
Decreasing Exposure to Pollen
Usually the best way to decrease exposure to an irritant is at its source. However, because airborne pollen can be carried for such long distances, it does little good to rid an area of an offending plant since the pollen can drift in from many miles away. In addition, most allergenic pollen comes from plants that produce it in huge quantities. A single ragweed plant can generate a million grains of pollen a day.
The next step to controlling at exposure is usually along the path from the source to the person. Often times ventilation is used to help control this, but do not rely on natural ventilation to reduce exposure to pollen. Natural ventilation improves indoor air quality, but it has several limitations. It is an inefficient way to dilute contaminants in a home because the air is not uniformly distributed and the flow cannot be controlled. Increased ventilation could also increase pollen levels if outdoor counts are elevated.1
To reduce the chances of pollen reaching people, keep the house clean. Pollen and other allergy-causing agents can be reduced, although not eliminated, through regular cleaning.
Find out what the daily pollen count is to know if it is safe to do things outdoors or open your windows. A pollen count, which is familiar to many people from local weather reports, is a measure of how much pollen is in the air. This count represents the concentration of all the pollen (or of one particular type, like ragweed) in the air in a certain area at a specific time. It is expressed in grains of pollen per square meter of air collected over 24 hours. Pollen counts tend to be highest early in the morning on warm, dry, breezy days and lowest during chilly, wet periods. Although a pollen count is an approximate and fluctuating measure, it is useful as a general guide for when it is advisable to stay indoors and avoid contact with the pollen.
Fernández-Caldas E, Trudeau WL. Environmental control of indoor biologic agents. J Allergy Clin Immunol. 1994;2(2):404-412.