In the vegetable cellar, vegetables just breathe, so they will produce a lot of carbon dioxide, and the density of carbon dioxide is higher than that of air, so carbon dioxide cannot be emitted in the vegetable cellar for a long time. So sometimes people will suddenly faint when they walk into the vegetable cellar. In order to avoid such a dangerous accident, we did a lighting experiment with burning candles before entering. If the candle goes out, it means that the carbon dioxide content is high and we can't get in.

So, the answer is: high carbon dioxide content; Lighting experiments should be carried out before entering the vegetable cellar.