Unless China finishes an ABM great wall that can shoot down most of US' second strike, it is kinda pointless to consider using nukes for either side, because it will just cause MAD.
If US uses poisonous weapons, they must still launch them from somewhere, unless they're all on ships. In the case of foreign aggression, China will fight back with unrestricted warfare anyways.
What will affect people more, some DU which 100 000s of US soldiers play around with and only end up with minor damages, or when Beijing punches in an aggressor country's IADS and then floods as many bombs as all the factories in China can make on them? Outrage over DU use is an useful marketing tool to justify PLA to fight without constraints, but it's not really a legit environmental concern.
A more legit concern is what will happen with the many US SSNs that will end up sunk if US uses them in an offensive. Those can actually cause environment damage.