Hi all! Not sure if this should be in the HVAC section as it is mostly related to building comfort but not directly to a HVAC equipment... Anyways, my uncle has been living in the same house since he purchased it in early '73. Since then, he never had any problems in the house (so he says) and apparently the moisture (RH) has always been in the 40-60% during winter, and possibly a bit higher during summers where he normally has to run the dehumidifier in the basement. Same applies for last summer, he ran his dehumidifier all day long in his basement to keep the humidity to a decent (healthy and comfortable) level (around 55%). He said he was removing about 5 gallons of water a day, that seems a bit much to me but he confirmed this is what his dehumidifier's tank capacity was.. Now, its the opposite. The RH stays around 20-25% and nothing will make it increase.. We tried to run the humidifier for hours, the RH goes to 40-45, then about half hour later after we shut the machine down, goes back down to 20-25%. The air feels really dry and we all have problems breathing. Our throats feel like it gets infected like when you are about to get a cold (itchy, runny nose, etc..). He is convinced that there is mold in his house. Could it be mold causing the RH issue and the breathing problems? One after the other, all the family members started to feel the effects of that strange phenomenon... I repeatedly mentioned that as far as I am concerned (and know), mold doesn't grow at such low RH levels. I do believe the "breathing" problems and cold-like symptoms are due to the excessively low RH. What can "suck" the moisture out that fast and consistently??? We have been searching for mold signs, no strange odors, nothing looks blackened.. Since last spring, he has developed some kind of tongue thrush his doctor is baffled about where it comes from.. Has anyone ever heard or experienced such scenario??? Any help is appreciated, my uncle and aunt will be more than happy if I bring them some interesting piece of info.. As usual, thanks to all!!