Years ago, my wife and I had a 8ft redwood hot tub, and we used chlorine as a sanitizer. We didn't care for the smell, but we didn't have problems either. A couple years ago, we bought a 8 ft modern spa, and the maker suggested bromine. We did not like the bromine smell, but worse, it caused itchy skin on my wife, and I developed eczema. I came across a mention of the Nature2 system on the web, and did not find any negative comments, and in fact, most reviews I read praised it as the next best thing since sliced bread. Okay, I bought four cartridges, 2lb of the Cense product, some test strips and a bucket of dichlor. I drained the tub, filled it up again halfway, and drained it again to eliminate as much of the bromine as possible. When I tested for bromine after filling it back full, it showed zero, and I added a new filter. I set up the Nature2 as per the instructions. Our spa is indoors, in a spa room attached to our house. Being cold at night, I was keeping the 2ft x 8ft window shutters closed. We normally keep the spa covered. The spa has an ozonator from the factory. When the spa was ready, I tried it out. There was no bromine smell, but contrary to what Zodiac claims about their product, we were getting what I can best describe as a metallic-ozone gas that was strongest when the pumps were on full blast. This gas caused extreme coughing, where I felt like I was going to cough up a lung, and sinuses that ran like a water faucet. After recovering from this, I next tried opening up one of the 2x8 windows for ventilation, and left the cover off, and ran the pumps at full blast to try to vent off these gases for several hours before getting in. That helped some, but still the gas became overpowering. I spoke to a spa-expert, who suggested that I try disconnecting the ozonator. I had already been thinking along those lines, and did so, along with venting the uncovered spa again. Again, the gas persisted. At this point, I am convinced that the Nature2 cartridge is to blame. I noticed that Silver Nitrate is listed as the active ingredient, and after some research discovered that it is toxic to mucous tissue, as well as skin and eyes. I Googled "Nature2 Respiratory Problems", and found a Material Product Safety Sheet produced by Zodiac as required by federal law. Reading that, they list silver, copper and aluminum as the hazardous ingredients. Further reading of that same sheet, mentions the health problems associated with these chemicals, the first aid required, and the need for respirators, etc. I notice they don't mention any of this stuff in their marketing, only that you are not supposed to ingest it. Between that sheet, and the noxious gas problem we are having, I am 100% convinced that this product is nothing close to the safe alternative they make it out to be. I have removed the Nature2 cartridge, and will now go back to either dichlor or bromine. Has anyone else experienced this gas problem, or did I just get a bad batch of Nature2? Compared to this, chlorine was a dream.