I PAT

I = P × A × T

Human Impact (I) on the natural environment equals the product of population (P), affluence (A: consumption per capita) and technology (T: environmental impact per unit of consumption).

This describes how our growing population, affluence, and technology contribute toward our environmental impact.

The equation was developed in the 1970's during the course of a debate between Barry Commoner, Paul R. Ehrlich and John Holdren. Commoner argued that environmental impacts in the United States were caused primarily by changes in its production technology following World War II, while Ehrlich and Holdren argued that all three factors were important and emphasized in particular the role of population growth.

The equation aids in understanding the various factors affecting human impacts on the environment. The technology factor can be confusing, as technology can be used both to save resources, and to exploit them further. However, generally technology is used to harness energy and increases impacts (ie, cars, pesticides, etc.).