has roger has stated be careful of mold and fungus build up. you know the old A/C units that were on the roof of commercial buildings where water calculated years ago. that's were legionnaires disease came from. we use the rent the big swamp coolers years ago but in GA it's really yo humid.
be careful and good luck, he is a link about legionnaires disease.
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http://www.infoplease.com/cig/dangerous-diseases-epidemics/legionnaires-disease.html