i use the candles outside the room so as the escaping odors will not be detected from someone that happens to be around there (not that there are people there, just in case)....this candle i use in 24/7 basis (of course i keep changing it when it burns out)......today i started burning a candle inside the growroom so as to consume the oxygen produced by the plants and in exchange release CO2 that in return will be used by the plants for their metabolic activities (photosynthesis) and thus grow bigger faster.....however it is not wise to burn the candle when lights are out because in that time the plants a)need oxygen and b)this would interrupt the dark cycle and propably as a result they would turn hermies