how do plastic bags damage the environment?

[888eddy]

i have been taught that plastic bags damage the environment because they take thousands of years to decompose.

this is not an explanation though. why is it bad that they take ages to decompose? i guess animals get trapped in them and some plastics are toxic but it would seem easy to put the plastic somewhere where animals wont get stuck and stop using the toxic plastics.

[mgb_phys]

Exactly - there is sometimes a rather circular argument in a lot of enviromental statements.
blown by the wind they become an eyesore and if they reach the sea they are a hazard to marine life (things eat them because tyhey look like jellyfish) but generally bureied in a proper landfill they are no worse than most of the other things put in a landfill.

Without wishing to starting a flame war - yes they do use oil to make, but not very much energy, much less than some other packaging. And a reusable heavy duty bag is better.
But the 'pastic bags are bad because they are plastic and plastic is bad because it makes plastic bags' is a bit simplistic.