Preanalytical and biological variability both have strong influences on test results of coagulation. Beyond age and gender, increasing emphasis is now being given to a variety of demographic and lifestyle variables, including ethnicity, smoking, diet, and exercise, which should be taken into account when interpreting laboratory data. However, there is strong emerging evidence that additional environmental influences, such as pollutants and environmental chemicals, might contribute in a major way to biological variability. A large body of epidemiological evidence now exists to support the view that air pollutants are responsible for shortened prothrombin time, and decreased factor VII, tissue plasminogen activator and platelet count; on the other hand, evidence also suggests that air pollutants may significantly increase fibrinogen, factor VIII, von Willebrand factor and platelet hyperactivity. Although it is impractical to develop reference ranges specific to the daily concentration of air pollutants, the potential influence of air pollution on results of coagulation testing should be recognised when interpreting laboratory findings.