Persistence is the ability of a chemical to stay unchanged in the environment for a long time. Persistent chemicals can result in concerns for the environment because:

• Emissions or discharges of the chemical into the environment are only slowly removed, so the amount in the environment can gradually build up to a level that can cause problems.
• The chemical can remain in the environment long enough to be transported long distances from the point of emission or discharge – possibly to more sensitive regions.
• Slow removal from the environment means that, even when emissions or discharges are reduced or stopped, environmental concentrations of the chemical will take a long time to fall back to background or non-effect levels.

The persistence of a chemical in an environmental compartment (i.e. in air, water, soil or sediment) is usually described in terms of its "half-life". Half-life is the time it takes for half of the amount of chemical to be removed from the environment. Half of the chemical disappears after one half-life; half of what is left disappears after a further half-life, leaving only one quarter of the original amount, and so on. This is illustrated in Figure 1.

After five half-lives, the amount of the chemical remaining is very small – only about 3% of the original concentration. For example, a chemical with a half-life of 6 days in water will have more or less disappeared after a month, but this will take a year for a chemical with a half-life of 70 days. The persistence of different chemicals can be judged by comparing their half-lives with each other or with that of a standard such as a common natural organic substance.

The actual rate of disappearance of a chemical from the environment will depend on the processes available for removing it. These processes, which will have different importance for different environmental compartments and in different parts of the globe, determine the effective half-life and thus the persistence of the chemical. Typical removal processes are:

• Biological breakdown, e.g. bacterial degradation in soil or sediment.
• Chemical (abiotic) breakdown, e.g. hydrolysis in soil, sediment or water.
• Transfer to a different environmental compartment, e.g. volatilization (evaporation) from water into the air.

In practice, the different processes for each compartment and their relative rates must be considered in order to assess the overall persistence of the chemical in the environment. These processes and their rates depend in turn on the nature of the environment as well as the intrinsic properties of the chemical. For example, both chemical and biological breakdown rates will depend on the temperature, moisture and pH (acidity) of the environment. Biological breakdown will also depend on the number and types of bacteria and other micro-organisms present.

A useful first step in assessing the persistence of a chemical can often be an evaluation of the relative importance of the three types of removal mechanism. Even a qualitative evaluation can provide a general understanding of how the substance will behave in the environment and the extent to which it is likely to be sufficiently persistent to merit consideration, at least in terms of this characteristic, as a POP.

One of the potentially adverse consequences of persistence is a build-up of environmental concentrations. This is illustrated in Figure 2, which shows how environmental concentrations change for chemicals with different half-lives. The maximum environmental concentration reached for each substance depends on its half-life. Substances with short half-lives (1 to 100 time units) soon reach a balance between emission and removal at a characteristic ("steady-state") environmental concentration. Once emissions stop, the environmental concentration drops back towards background levels. On the other hand, for substances with long half-lives, the environmental concentration keeps on increasing. Even when emissions stop, the concentrations fall very slowly.

In practice, emissions and discharges of a chemical will be different to each environmental compartment. Some chemicals may be mostly emitted to air; while others may be mostly discharged to water. Similarly, in each of these environmental compartments, the removal processes and different rates of removal will vary depending on the characteristics of the chemical. The properties of the chemical (e.g. its solubility, its volatility and its polarity) will determine its tendency to move from one environmental compartment (e.g. water) to another (e.g. soil, sediment or air) and will influence its susceptibility to biological and chemical breakdown.

Evaluating the persistence of a chemical thus requires knowledge of:

• The intrinsic physical and chemical properties of the chemical as they affect chemical or biological breakdown within an environmental compartment, e.g. hydrolytic stability.
• The properties of the chemical as they affect transfer – and thus distribution - between environmental compartments, e.g. volatility, solubility, strength of binding to soil particles.
• The properties of the environmental compartments concerned (e.g. pH and salinity of water).
• The interval between emissions or discharges

Any use of half-lives as criteria must take account of all four types of data. The first two points relate to chemical-specific characteristics represented by the chemical’s effective half-life in the environmental compartment. The last two points highlight the very different numerical values of half-life that might trigger concern – depending on the environmental compartment considered, its properties, and the associated pattern of emissions.

R.J. Larson and C.E. Cowan. 1996. Quantitative Application of Biotransformation Data to Environmental Risk and Exposure Assessments. Env. Tox. and Chem. 14(8): 1433-1442.