Whether they are grown in soil, grow rooms or green houses, plants need air, water and sunlight. Growers therefore need to ensure that plants receive adequate amounts of these essential inputs. But it is equally important to take adequate measures to ensure that plants do not fall prey to pests and parasites that are always present in the air. In the controlled conditions of greenhouses and grow rooms, one way to do this is to use filters to keep out molds, bacteria and other organisms. But filters come in many varieties, so how do you go about deciding which one is the best your money can buy?