The A/C compressor saps about 5 hp, about the same as the power steering pump & alternator combined. If you think about it, the engine is only generating probably twice that at idle, so no wonder the rpm drops when the A/C is turned on. The most recently used refrigerants (R134a & R1234yf) do not deplete the ozone, they do however contribute to greenhouse gas production. The previously used refrigerant, R12, was used up until the early 90s & that did contribute to ozone depletion. "Freon" is a trade name BTW, the same way you'd call a vacuum cleaner a "Hoover" or "Sellotape" sticky tape.
Going slightly off-topic, I work as a contractor for a large pub chain & their policy is to open all external doors in the pub when the outside temperature goes above 21 deg. C with the aircon going flat out. Go figure
