Ok, here's how the health rating could work...
For terminology purposes, health is the inverse of pollution. A city's total health + total pollution always equals 100.
Pollution causing stuff:
1 pop = 1 pollution
square root of total shields produced is pollution
certain buildings cause pollution, such as factories, power plants, city walls (encourages overcrowding)
certain buildings reduce pollution, such as aqueducts, sewers, recycling centres
Total pollution = % of *excess* food production lost.
If pollution > 100 then:
(pollution - 100) = % of *base* food production lost. All excess food production is automatically lost.