Im not an expert, but Ive read that HIDs are some of the hottest lights. Besides need ventilation for the heat, you need to have a fan or some wind to help make the stem strong, w/o any air flow, the stems will not be able to keep up w. the weight of the plant. Also, ive read that the leaves release toxins by the passing of wind over the leaves. No matter what you need ventilation, I have fan going the whole time so that the stem is very strong and your plant won't topple over when it's gets taller.