Producing CO2 for me a propane burner isn’t going to work in a 4×4 tent. A CO2 tank is bulky, pricey and needs to be taken to get refilled. So I says to myself, self what about candles? Unscented candles give off CO2 and water vapor, so I found 8 hour burning tea light candles unscented and tiny little lanterns, they are made of glass and metal and used for like weddings for on the table. So anyway the little lanterns are less than 4 dollars and will burn one tea light candle so I got two for each of my tents. Each candle adds about 400 ppm. So one candle with ambient 400 gives you 800. burn two and get about 1200.