Single-stage impactor for pollen and fungal spores monitoring: what does it really capture?

Since the 70s the mostly used instrument to monitor the concentration of atmospheric pollen and fungal spores was the Hirst Volumetric Spore Trap, recommended by the International Biological Program (1964-1974) through the Integrated Research Program of Aerobiology (1970-1971) and later widely adopted by National networks of airborne pollen monitoring. The Hirst spore trap is a single-stage impactor were the particles collector is a tape or a microscope slide. Despite the spreading of this type of sampler, there are still relevant problems that limit its reliability as measuring instrument. These problems can arise both from producers and users. The main requirement of this instrument, like with other kind of aerosol samplers, is represented by the capture (or collection, or sampling) efficiency, which depends on size, density, shape and aerodynamics of the particles being sampled. In other words, capture efficiency (E %) is the ratio between the particles present in the flow upstream the collector and the particles which will be deposited. As regards the Hirst type sampler, it does not seem that, up to this moment, the sale of this instrument is accompanied by the capture efficiency certificate. Generally the volumetric spore trap is used to measure the atmospheric concentration of pollen grains and fungal spores in outdoor and indoor ambient and the capture efficiency tests are usually carried out with particles having aerodynamic diameter of 2, 10 and 50 microns. Therefore these measures cannot be done in open air, but must be carried out in laboratories equipped with solid aerosol generators and suitable measuring instruments.