The effects of electromagnetic fields can be distinguished in acute and chronic effects. Acute effects occur directly after short exposure, whereas chronic effects only occur after a longer period of time under constant exposure of several days, weeks or years.
To examine chronic effects on humans, prospective epidemiologic studies (so called cohort studies) are conducted in which a group of people are observed for a longer period of time to assess whether an increased health risk is caused by a specific exposure. This type of study is appropiate to investigate frequently occurring diseases but for rare diseases (e.g. childhood leukemia with an incidence of 5 cases in 100.000 children) it is almost impossible to recruit the required number of participants.
In animal studies, chronic effects are studied under controlled conditions over longer periods of life, and even over several generations. Here, rodents are often used with average life expectancy of 1.5 to 3 years.
The term "long-term study" is not scientifically precisely defined and the duration of such a study depends on the species (e.g. mouse or human) being examined.
Acute effects in humans can be investigated in provocation studies in which subjects are usually exposed to a defined electromagnetic field for a short time (seconds to several hours) in the laboratory under controlled conditions. Similarly, acute effects can be studied in animal experiments. In these experiments, the animals are often restrained to better control the exposure conditions or for local exposures. Cell and tissue investigations are usually performed to investigate acute effects under very controlled and defined conditions.