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Interpreting Information on Health Effects of Substances in the Scottish Pollutant Release Inventory: Exposure and Dose

This information has been produced in partnership with Health Protection Scotland. Its principal aim is to improve the health of the Scottish population by providing the best possible information and expert support to pracitioners, policy-makers and others on infectious and environmental hazards.

The Scottish Environment Protection Agency (SEPA) is responsible for ensuring the protection and enhancement of Scotland's environment and is the regulatory authority responsible for licensing and controlling emissions from industrial processes in Scotland. SEPA uses its regulatory powers to ensure that industrial processes use the best available technology (BAT) to reduce pollution and that internationally prescribed safe limits for substances in the environment are not exceeded at any time. The sites appearing in the Scottish Pollutant Release Inventory(SPRI) are also obliged to report the total amounts of each substance emitted to air or water over 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 concentrations in the environment. Thresholds above which mass emissions of each substance must be reported have been prescribed using knowledge of the pollutants (toxicity, transport and persistence in the environment) to relate mass emissions to "significant" environmental concentrations.

Alongside concern for the environment, a primary aim of emission control is to ensure that no one receives a dose from exposure to a particular substance that might be damaging to their health. We are familiar with the idea of a safe “dose” when we think about medicines. However, when applied to substances we might take into our bodies from our environment the issue is more complicated. In most cases it is not possible to accurately monitor what we are actually taking into our bodies in the air we breath (respiration), through our skin (dermal contact) or in the food or water we consume (ingestion). The approach taken to ensure that we are not exposed to levels of any substance which could cause harm to our health is to set a concentration limit (or standard), usually expressed as a mass per unit volume of air, water etc. This limit is based on knowledge of the toxicity of the substance and the duration and nature of exposure, and ensures that environmental concentrations do not result in an unsafe dose for the population or any individual within it. Environmental standards established in this way always incorporate a significant safety factor.

Environmental standards vary considerably for different substances. This is because the doses which could cause harm vary: some chemicals can have health effects at very low levels; whilst some everyday substances, ordinarily considered very safe, would only harm us if they entered our bodies in quantities far beyond those we normally experience.

Many factors apart from the concentration of a substance in the environment around us also affect the dose we receive. For some substances in the air, the dose we receive will be greater if we are exercising or exerting ourselves at the time of exposure. Our habits and activities may influence the dose received. It is also possible to be exposed to the same substance through various routes: at home, in the workplace, or outdoors in towns or the countryside, as well as through the food we eat and the water we drink. The route of exposure may influence where the chemical builds up in the body. Also, in some cases, the effects of a chemical may be reduced or even multiplied by the presence of another different chemical.

Personal factors like age, sex or body weight, or whether a person has pre-existing illness can also affect the dose received or the harm an exposure may cause.

In summary, the dose of any specific substance received by an individual is dependent on a number of factors including;

• the concentration of that substance that is present in the air, land or water
• the amount of time for which an individual is exposed to that substance
• how easily that substance is taken up by the body
• how the body deals with that substance, e.g. through metabolism, excretion etc.
• personal factors about the individual e.g. age or healthiness.

A more in-depth commentary on the interpretation of health effects of environmental exposures will be made available on the website Health Protection Scotland  in the near future.