Comment by Terr_

In some cases it still is, but we need to emphasize the exceptions, which can be rather serious.

For example, we can hardly "dilute" CFCs or CO2 any more than we did, by putting them into a whopping 5.15×10^18 kg of the entire atmosphere of the Earth. Yet both still cause bad things, because there's no (sufficient) process to break them down or move them to a safe state.