Guest editorial

What do pollen counts mean?

Every evening during the summer the meteorologist reportsthe pollen and mold counts on the local news. I assume thatthis is supposed to help me decide whether to plan outdooractivities if I have allergies. The counts often are accompa-nied by an interpretation such as “that sounds bad” or “allergysufferers beware,” with an occasional joke about smut whenUstilago species predominate; however, the meteorologistgenerally does not tell me what those counts actually mean.1

Presumably, the higher the count, the more likely I am tohave allergy symptoms, in much the same way as higherrainfall probability is associated with a greater likelihood thatI will get wet.

Weather conditions can and do vary over short distances inresponse to geographic features. Lake-effect snow in Buffalo,NY, is a classic example of this. Yet, it is not clear how pollenand spore exposures vary with geography, and neither is itclear whether such variance is of any practical importance toallergy patients. The weather forecaster tells me when therain is likely to fall so that I can more accurately plan mydaily activities; however, pollen counts come with no suchdiurnal information.

In the article by Katelaris et al,2 the authors measuredpollen counts using Burkard samplers at 3 sites located 15 kmapart along with weather conditions and vegetation differ-ences in an attempt to identify relationships among the vari-ables. Weather data were obtained from stations located neareach collection site, giving local weather to go along withlocal pollen counts. Although temperature was not related tototal pollen counts, rainfall clearly reduced the counts. Thepresence of local plants also had an effect on the reportedcounts. Greater uniformity was found for pollens produced ina diffuse area, whereas those produced by local sourcesshowed substantial variability. Significant differences werefound between 8% and 17% of the time in a 30-km radius,leading the authors to conclude that local variation couldsignificantly affect patient exposure.

We measure pollen and spore counts as a proxy for totalairborne allergen exposure, yet it has been estimated that upto 50% of allergen exposure occurs in the form of a submi-cronic bioaerosol.3 Direct measurement of this bioaerosolmight provide a more accurate estimate of total exposure, butsuch measurements are not generally available. The problemwith pollen counts is that pollen collection and enumerationis very labor intensive. Ideally, each weather station in acommunity would measure weather factors and pollen locallyin real time. This is simply not feasible given the technologyavailable today. Pollen counts represent collections made atleast 24 and sometimes 48 hours earlier, and they rarely

permit an estimation of what time of day a given pollen grainwas present.

All of this begs the question of how airborne pollen affectshealth. The authors correctly point out that we simply do notknow how much pollen exposure it requires to elicit a certaindegree of symptoms. That depends on the duration and routeof exposure, the sensitivity of the patient and their targetorgans, and how symptoms are measured. Katelaris et al2 alsopoint out that clinical studies generally rely on pollen countsto test the effect of treatments on elicited symptoms. Thereality is that pollen exposure and the resulting symptoms arerelated to many independent factors that are not consideredby reports of local pollen counts.

Collection factors that affect pollen counts include geog-raphy, location, elevation above the ground, time of day, typeof collection device, duration of collection, the skill of thecounter and how fatigued he or she is, the quality of themicroscope, and weather conditions,4 including the presenceof El Nino that year.5 Health effects depend on the durationof exposure, whether the exposure is in the form of pollengrains or bioaerosol, the potency of the bioaerosol, the pres-ence of specific IgE antibodies, the sensitivity of the targetorgan, and the person’s subjective experience of symptoms.6

Given all of this uncertainty, pollen counts alone provide mewith little guidance when I decide whether to participate in anoutdoor activity during allergy season.

So can pollen counts lead to reasonable recommendationsfor the lay public? Since the public clearly desires this type ofinformation, we need to suggest strategies for reducing ex-posure when appropriate. The current approach of monitoringpollen and spore exposure from a central station is the onlypractical one at this time. Although it is not practical to placean air sampler every 160 m (as suggested by the distance for90% pollen fallout from a focal source),7 a periodic survey ofthe types of pollinating plants in the region receiving reportsfrom a particular station could be made to supplement thedaily counts, along with recommendations for location-spe-cific adjustments. On days when rain is expected, the publiccould be reminded that rainstorms usually are accompaniedby a burst of pollen associated with the wind front followedby a reduction that lasts for several hours after the rainfall.8

Because the diurnal variation in pollen and spore countshas not been well characterized for most genera, recommen-dations can be made only for those for which data are avail-able. Ragweed, for example, is released from sources in themorning but generally reaches peak concentrations at noon.9

A moving average count may provide a better estimate of

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pollen and spore exposure on a particular day than the actualcount.

Despite the complexities involved in using communitywidebioparticulate counts to estimate personal exposure, informedallergy patients can act to reduce their allergy symptoms. Dailyreports of the counts also may encourage community efforts toreduce exposure to certain types of pollen. Ultimately, the mainbenefit derived from pollen and spore counts is the public’senhanced awareness of the importance of allergic diseases andfamiliarity with the aeroallergens that trigger them. This in-creases the likelihood that allergy patients will seek medical helpwhen they need it and that physicians will be familiar with thehealth effects of the counts and be more prepared to care forallergic patients when they are seen. Ultimately, pollen andspore counts benefit patients, physicians, and the communities inwhich they live.

JAY M. PORTNOY, MDChildren’s Mercy HospitalKansas City, Missouri

REFERENCES1. Frenz D, Goswami A, Murray L, Lince N. A survey of televi-

sion meteorologists about their sources for and understanding ofpollen counts. Ann Allergy Asthma Immunol. 1998;81:439–442.

2. Katelaris CH, Burke TV, Byth K. Spatial variability in thepollen count in Sydney, Australia: can one sampling site accu-rately reflect the pollen count for a region? Ann Allergy AsthmaImmunol. 2004;93:131–136.

3. Barnes C, Schreiber K, Pacheco F, Landuyt J, Hu F, PortnoyJ. Comparison of outdoor allergenic particles and allergen lev-els. Ann Allergy Asthma Immunol. 2000;84:47–54.

4. Barnes C, Pacheco F, Landuyt J, Hu F, Portnoy J. The effect oftemperature, relative humidity and rainfall on airborne ragweedpollen concentrations. Aerobiologia. 2001;17:61–68.

5. Freye HB, King J, Litwin CM. Variations of pollen and moldconcentrations in 1998 during the strong El Nino event of1997–1998 and their impact on clinical exacerbations of allergicrhinitis, asthma, and sinusitis. Allergy Asthma Proc. 2001;22:239–247.

6. Portnoy J, Barnes C. Clinical relevance of spore and pollencounts. Immunol Allergy Clin North Am. 2003;23:389–410.

7. Frenz D. Interpreting atmospheric pollen counts for use inclinical allergy: spatial variability. Ann Allergy Asthma Immu-nol. 2000;84:481–491.

8. Suphioglu C. Thunderstorm asthma due to grass pollen. Int ArchAllergy Immunol. 1998;116:253–260.

9. Barnes C, Pacheco F, Landuyt J, Hu F, Portnoy J. Hourlyvariation of airborne ragweed pollen in Kansas City. Ann Al-lergy. 2001;86:166–171.

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