Soil and human health: an epidemiological review

Two different approaches have been used to study relationships between soil and human health: aggregate- and individual-level. Aggregate-level is the primary approach used in the geosciences and broadly relates spatial soil characteristics to geographic incidence of disease. However, this may not be appropriate for a wide range of exposures and disease outcomes. For example, many diseases with long latency periods are associated with early life or cumulative exposure to the causative agent, rather than an individual's current geographical location. Public health scientists and epidemiologists often refer to aggregate-level studies as being 'hypothesis forming', and consider individual-level approaches as the 'gold standard' for investigating causes of human health outcomes. This paper investigates the appropriateness of individual-level study for disease outcomes associated with soil by reviewing the weight of evidence from individual-level studies that have included exposure to soil as a risk-factor for disease. The majority of these studies are very specific, inspired by explicit case reports and medical records. The review showed that exposure to soil was implicated mainly in the spread of enteric parasites, but also in the incidence of certain cancers, bacterial infections, and mycoses. There was little evidence for specific soil contaminants as causative agents of disease at an individual level but there was also little evidence contrary to this. Further individual-level studies into soil:human health scenarios are required in order to derive more appropriate dose-effect relationships for regulatory science and risk assessment.