I have put my GMC-300 outside, connected to a Raspberry Pi, and this yellow-green pollen coated my equipment. The GMC-300 shot up over 500 CPM for about 3 hours and then drop back down to around 25 CPM. I did not touch anything because I was not at home. Here is the chart from NETC.com software that I was running at the time.
I have cleaned off the yellow-green pollen from my outside table and have connected a second Geiger counter to see if this happens again and will both units catch the radiation. The second unit is a GMC-200
I am tied into Netc.com and using their software code. Go to NETC.com and check it out. You can buy the code for $19.95 without the 8 gbyte chip. It is in beta testing now. Email staff@netc.com