It’s pollen season, and where I live in Atlanta Mother Nature has given everything with the misfortune to be outside a healthy dusting. Now that my car has the same greeny-yellow slime color of an abandoned pool I am resolved to do some digging into the phenomenon. However, I feel pollen is fairly well known natural event. It’s part of plants making baby plants and bees are involved. What else is there to know right? Well for starters, how are pollen levels measured? Pollen levels being the standard metric by which news organizations inform drugstores to stock up on tissues and anti-histamine. This concept has gotten my engineer-sense tingling…unless that’s just… Oh geez…
Just kidding. My immune system is like an impregnable fortress made of Dwarven forged mithril (the good stuff), but if you are one of the many people who suffer from pollen allergies (known collectively as allergic rhinitis AKA hay fever) you should take solace in the fact that science is on your side. Right now, on a rooftop somewhere in your area is a greased up rod spinning around in a tube that is sucking in air and pollen. This doohickey has the understandable but perhaps uninspired name of a “rotorod”. Air is sampled for a fixed interval, usually for one minute out of every ten minutes for a twenty-four hour period, and any pollen grains in the sampled air are stuck to a layer of silicone grease on the rotorod as they pass over its surface. After the sampling period a special staining solution that contains a dye designed to adhere preferentially to the surface of the pollen grains (and not for example on mold or dust also in the air) is put onto the rotorod localized pollen, and then a fixed area of the rotorod surface is observed under an optical microscope. By utilizing the fact that the sampling rate, volume of air sampled and observation area are all fixed the number of pollen grains on the observed area of the rotorod can be used to calculate the average concentration of pollen grains floating around in the air.
If your response to that last paragraph was, “Wait, somebody has to physically count every single pollen grain collected during the test AND because they are a good scientist and want to minimize the scatter in their data took multiple tests? I thought you said science was on MY side!” then I am right there with you friend o’ mine. Sounds like a lot of work, especially when automated particle counters exist to constantly measure the concentration and size of particulates in the air. These particle counters use either the blocking of light (for particles larger than about 1000 nanometers) or scattering of light (for particles around 50-500 nanometers) and with similar methodology to the rotorod and microscope method (known volume of sampled air at fixed sampling intervals) these devices can determine total particle concentrations and size distributions for a given volume of air. This type of sampling technique is commonly used to monitor the “cleanliness” of clean rooms used for semiconductor and satellite fabrication where even minor levels of contaminants can cause device failure. The best clean rooms have as few as one or less particle with a diameter of 500 nanometers in every cubic foot of air. As someone who has spent some time as a famous clean room technician (that’s me in the background!), and considering the fact that normal “dirty” room air has over a million particulates of the same size for every cubic foot, I can tell you these are some pretty clean rooms.
One very important reason for not using this technique is the fact that pollens of different varieties imply different allergic responses for different people. Thus reporting a total pollen count or grain size distribution might not be an effective method to inform the public about the likelihood of a runny nose in their future which is the whole point of monitoring pollen levels in the first place. As a consequence, a trained specialist must identify both the species and concentration of pollen grains with the method described above which results in the reported “pollen count”. The myriad of sizes and shapes among different pollen grains, which macroscopically have very similar appearances, is shown in this false color image taken in a scanning electron microscope shown to the left. The original grey scale image (all SEM images are taken in grey scale) can be found here with call-outs to identify the pollen grains.
All of this finally brings us to the featured topic: the pollen count. From some sources only a value of high or low, moderate or severe or something of this nature is given, but this belies what we should now understand is a much more detailed piece of data from a very sensitive measurement system: a trained scientist. One source reported that the pollen count in Atlanta hit a record at over 9000 grains per cubic meter which seems like quite a lot, but not so bad when you consider the recent panic in Moscow when clouds of pollen from trees were so thick people thought they were toxic chemical fumes. Bonus points if you can spot the trained scientist in the video from Russia. With that knowledge, I’m feeling a little better about my car.