Understanding pollen and mold counts

For the 35 million Americans who have seasonal allergies, rising pollen counts in the spring bring symptoms that can ruin a time of year the rest of us enjoy immensely.

Allergy symptoms are not only annoying, but for many people interfere with their ability to work, perform at school, or even sleep. Understanding the significance of pollen and mold spore counts can help better manage symptoms.

Pollens and molds represent the clinically most important outdoor allergens. Researchers are interested in pollen and mold sampling because these airborne allergens occur in easily identifiable units.

Attempts at counting pollens go back over 100 years. Initial investigators collected particles on an adhesive coated slide. However, variables such as wind speed and particle size changed the number of particles coming to rest on a slide, affecting the accuracy of this method.

More recently, researchers have developed volumetric techniques that measure the concentration of pollen grains or mold spores in the air. This gives them a better estimate of how much pollen people in the sampling area are being exposed to.

Two of the more popular volumetric samplers used in this country over the past few years are the Rotorod sampler and the Volumetric Spore Trap. The Rotorod sampler uses small adhesive coated rods that intermittently spin through a certain volume of air so that the particles in that air stick to the rod. At the end of 24 hours, the particles are identified under a microscope and the concentration of particles per cubic meter of air is calculated.

The Rotorod sampler does a good job collecting pollens and larger molds. However, smaller mold spores are not as efficiently collected resulting in low estimates for counts of certain mold spores.

Volumetric Spore Traps draw air into the sampler at a given rate. The particles in the air land on an adhesive coated microscope slide. After a period of sampling, usually one day, the sample is stained and the pollen grains and mold spores are identified and counted, and the concentrations in the air are calculated.

Regardless of the method used, the reported counts reflect the average count for the sampling period, not a real-time count determined at the time of the report. For example, a pollen count reported on Monday probably represents the average count for the 24-hour period ending at some time on Monday morning.

Because various pollen and mold counts vary over the course of the day, the average count gives us an estimate of overall exposure. At certain times during the day, the concentration of various particles could be temporarily higher.

The American Academy of Allergy Asthma and Immunology (AAAAI) organized the Aeroallergen Monitoring Network, which has compiled pollen and mold counts for more than 30 years. The network was established to further the science of allergy, and to contribute to the information available to physicians for the diagnosis and treatment of allergic disease.

The Network has reported pollen and mold spore counts to the public and the media since 1992 through the National Allergy Bureau, a service established by the AAAAI. Member stations report pollen and mold counts to the NAB which releases reports to interested media outlets and to the public through the AAAAI web site (www.aaaai.org/nab). Results are reported as total tree pollen, grass pollen, weed pollen, and mold spore counts per cubic meter with comments about their relative amounts. This information allows allergy sufferers and their physicians to correlate symptoms and causing agents. 

Accurate forecasts of future counts would allow people to adjust activities on days with predicted high counts. Forecasting involves having accurate counts from previous years at the involved site and taking into account meteorological data.

Researchers involved with the network are working on such predictive models, but want to prove their reliability before making such predictions available to the public. Until reliable forecasts are available, people with symptoms will have to rely on trends in recent high pollen counts to alert them to take appropriate precautions regarding avoidance.