SEPA regulates emissions from industry by permitting sites to emit substances, set at levels, which take into account the best available technology (BAT) to reduce pollution, while also ensuring that internationally prescribed environmental quality standards (EQS) are met. EQS are designed to ensure that concentrations of each substance in air, land and water do not reach levels which could be damaging to the environment or human health. All sites licensed under the Pollution Prevention and Control (PPC) legislation are periodically reviewed to ensure that they are using the latest and most appropriate abatement technology and that emissions do not exceed any new EQS.

Industrial sites listed in the Scottish Pollutant Release Inventory (SPRI) are also required to report the total amounts of a substance that they emit to air or water during the period of a year. This amount is known as the “annual mass emission”. Annual mass emissions alone are not necessarily directly related to concentrations being emitted at any particular time and cannot be used to directly predict the resulting concentrations in the environment. Thresholds above which mass emissions of each substance must be reported are prescribed using knowledge of the pollutant (its toxicity, transport and persistence in the environment) to indicate what mass emission may give rise to “significant” environmental concentrations. Sites reporting to the SPRI all have total annual mass emissions above the reporting threshold for a particular substance, but this does not imply that these sites are in breach of their licensed emission concentration limits at any time during that year. High annual mass emissions are often due to the large size of the industrial process, where relatively low concentrations are released in very large flows of air or water.