Risk assessment involves the evaluation of the likelihood of harm and its consequences for populations or individuals. Risk control requires the prioritisation of risks and the introduction of measures to prevent or reduce the harm from occurring. It is often assumed that an assessment of risks is scientific and objective whereas risk control is less straightforward because it combines the findings of risk assessment with other inputs, such as cost, risk perception, availability of technologies, etc. where there is more room for subjectivity. In practice, it is difficult to separate the two processes and assess risks without making assumptions. Consequently risk assessment becomes a mixture of science and policy and a tool for extrapolating from statistical and scientific data to obtain a value which people will accept as an estimate of the risk attached to a particular activity or event.