Counterfactual threshold for ozone (Jerret et al 2009)
cf_o3
An object of class numeric
of length 1.
Jerrett, M., Burnett, R.T., Pope III, C.A., Ito, K., Thurston, G., Krewski, D., Shi, Y., Calle, E. and Thun, M., 2009. Long-term ozone exposure and mortality. New England Journal of Medicine, 360(11), pp.1085-1095.