Plastics should be avoided because-
These are non-biodegradable materials and hence require a very long period of time to decompose.
They cannot be disposed easily and hence, get accumulated over a period of time, and cause water and soil pollution.
They also cause clogging of drains, leading to many problems.
Plastics on burning cause air pollution.
They also adversely affect the lives of aquatic animals and animals like cows etc. who swallow the plastic unknowingly.