Modifiers are fixed in time could be relaxed to allow modifiers that change in time much more slowly than does the air pollution, such as age, chronic disease, and some long-term treatments
Because of the baseline seasonal pattern of mortality, the additional non-pollution related modifiers of risk by predisposing condition might be captured by a sine and cosine term with 365.24-day period
suggested the effect modification for people with COPD on air pollution associated out-of-hospital coronary death