Final answer:
A disadvantage of using flexible plastic films in greenhouses is the potential health risks associated with plasticizers, which can migrate from the plastic into the environment.
Step-by-step explanation:
One disadvantage of using flexible plastic films as a covering for greenhouses is that they can contain plasticizers, which are additives that increase the plastic's flexibility. These plasticizers are often small enough to be diffusible and can pose potential health problems as they migrate into the environment. This is particularly the case with polymers such as polyvinyl chloride (PVC), which is used in many flexible vinyl materials. The odors associated with these materials are evidence of their ability to escape into the surrounding air.