Data gaps must also be acknowledged, and approximated if at all possible. When approximating, conservative estimates should be made -- in other words, when assessing an individual's exposure to second-hand smoke, assuming exposure over a 24-hour period (the maximum possible exposure to the environment) rather than the minimum, is usually preferred. or, in the case of childhood exposure to lead paint, even if this cannot be confirmed, if the individual lived in an area where residency in lead-painted accommodations was likely, exposure to the toxin may need to be factored into an analysis of the subject's health status. Conservative estimates seem like a necessary, compassionate,...

9/11 was an extraordinary, unprecedented event, and data collection at the point of time was virtually impossible, yet it had very real effects upon emergency responders, given their higher rate of manifesting respiratory illnesses. Creating an approximation of exposure seems necessary for both ethical as well as environmental reasons, to ensure these responders are compensated for long-term, job-related health consequences.
Works Cited

Prezant D.J., Weiden M., G.I. Banauch, et al. (2002). Cough and bronchial responsiveness in firefighters at the World Trade Center site. New England Journal of Medicine.