Many of the rules in the Province of Ontario allow a facility to emit pollutants with no APC equipment at all at as high a rate of the pollutant as it wants as long as it performs some basic modeling to demonstrate that the worst-case maximum ground-level concentration of the air toxic that the public is potentially exposed to does not exceed its health-based exposure concentration standard, taking into consideration other emission sources of that air toxic in the area. This rule also forbids the operation of any process – no matter how extensive the controls – if the impacts of its emissions after those controls exceeds its health-based standard. This is a way to protect an area’s residents in terms of public health. In the US, there are few laws like this. In the US, we believe in consistency: treat all similar facilities alike and focus control on the emission rate or percent control. What is a reasonable degree of control of toxic air pollutants to be expected for a given operation from a public health situation and not too expensive to strongly impact the facility? But such rules ignore health-based standards. One can (and does) have situations where sources of air toxics meet the percent control or mass emission standard, but their impacts indicate a potential public health problem anyway despite compliance with the rule. Perhaps because of geographic or meteorological factors from modeling. On the other hand, such thinking may cause facilities to spend a lot of money to control emissions even as their health-based standards can still be met with fewer or no controls. Consistency and operation-based regulations or health-based public protection? Which type of rule do you think is best or more fair for society?