1) Plastics are non-biodegradable so they take centuries to decay depending upon their composition. When the plastics are disposed indecently, it gets washed away into different water reservoirs. Thus it gets clogged in the waterways and float on reservoirs, hence polluting and making them undrinkable and unusable. Plastic contamination affects adversely to the environment thus harming humans, animals and plants.
2) Plastics have low melting point, so they can't be used as protective barrier for furnaces. Plastics like polystyrene, acrylics, polyethylene and nylons are highly flammable but as they are commonly used in packaging and in home and office appliances so this makes a way for a easy fire hazard.
3) Plastics have a short useful life-time as compared to other materials. This short life-time usage of plastics results in their piling up of unwanted garbage in the office, home or waste yards. Although some of the plastics are recycled but most of them remain uncollected in dump sites and pollute the environment.
So we should avoid plastics as far as possible.