A substance is generally considered a pollutant when it is present in the environment at levels that cause adverse effects. These effects can impact human health, damage ecosystems, or disrupt natural processes. Typically, a substance is deemed a pollutant if it exceeds a certain threshold, is non-naturally occurring in that environment, or persists for an extended period, leading to cumulative harm. The exception to this might be when the substance is naturally occurring and present at normal levels, or when its introduction does not cause significant harm.