Incommensurable is a good word. It means two things can't be measured with the same ruler, like 5 minutes and 25 kilograms. It came to mind when I read this account of the London cycle scheme.
Because of the way the scheme works, every journey by every cycle in the scheme is logged into a database. So you can work out how much polluted air the cyclist has inhaled. Then you trade off the extra particles inhaled against the benefit of the extra exercise. And of course more exercise is good for you.
But cyclists get killed, particularly, it seems, young women. And for me those deaths are incommensurable with the notional, statistical, modelled benefits.
I know that's not how you're supposed to think in public health.