Examples of these include non biodegradable bottles and heavy metals. Since these pollutants do not biodegrade or become absorbed, they simply accumulate over time. As the accumulation of stock pollution increases with the continual release of the pollutant, so does the damage from this pollution. The damage from this pollution often can cause health related problems and environmental problems like that of acid rain.
To find the efficient allocation of stock pollutants take for instance the production of commodity X. The production of commodity X generates a proportional amount of stock pollutant (whatever that may be). The pollutant can be reduced however its at a cost equal to that of reducing an equal amount of production of commodity X. The damage from the stock pollutant is equal to the amount of accumulated stock over time. Therefore as long as the stock of pollutants exists the damage persists.