Materials and methods
Resistance is a system-level metric used to estimate the balance between environmental exposure and biological sensitivity. There are a number of potential variables and ways to calculate it. Exposure and sensitivity can be measured by single or multiple variables, having different units, and often needs to be weighted, standardized, or normalized for appropriate comparisons. Moreover, resistance can be calculated as either the difference or ratio between exposure and sensitivity. Therefore, a number of these possibilities were evaluated and described below.