1. "negative" is lower than atmospheric pressure (that is often said of one atmosphere) of the gas pressure conditions, the use of negative pressure is very common, it is often a part of the space so that a negative pressure or vacuum state appear, can use everywhere the atmospheric effect for us. For example: the use of vacuum heat the workshop smoke odor emitted. Resolve bad ventilation plant workshop and hot issues.

2. "positive pressure" is higher than atmospheric pressure (that is often said of one atmosphere) of gas high pressure gas state. For example: the forced cold air cooling equipment (air conditioning, fans, chillers).

3. The distinction between "positive and negative" is just the opposite: a positive pressure is blowing, vacuum exhaust.