Counterfactual threshold for ozone (Jerret et al 2009)
cf_o3An 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.