The study compares the course of the spore season of Alternaria in Bialystok, Cracow, Olsztyn, Piotrkow Trybunalski, Sosnowiec, Szczecin, Warsaw, and Wroclaw in 2020. The investigations were conducted using the volumetric method. Alternaria spore season was defined as the period in which 90% of the annual total catch occurred. The Alternaria season started first in Bialystok on May 19 and lasted on June 28 in Wroclaw. The highest airborne concentration of 1052 Alternaria spores/m3 was noted in

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... roclaw on July 30. The highest annual sum of Alternaria spores (SPI) was observed in Warsaw (19 390 spores) and the lowest in Bialystok (5769 spores). Most days, the threshold concentration inducing symptoms in allergic persons occurred in Piotrków Trybunalski, Szczecin, and Warsaw.