I'm more immediately concerned about the polluted products they're exporting into our country, such as lead-contaminated toys as just one example.
As for polluting the environment, well we do plenty of that ourselves, not just here but in other people's countries by American companies operating there (including China) because environmental laws are more lax (and have cheaper labor); the maquiladoras along the Mexican border come to mind as I live in a border state. What they've done to the Rio Grande River is horrible.