This design can also be used to develop estimates of the impact
of TSP concentrations on life expectancy. Specifically, if the
Huai River policy only influences mortality through its impact on
TSPs, then it is valid to treat Eq. 2a as the first stage in a twostage
least-squares (2SLS) system of equations. An important
appeal of the 2SLS approach is that it produces estimates of the impact of units of TSPs on life expectancy, so the results are
applicable in other settings. The second-stage equation is