1. systematic increases in concentration of substances from the earth's crust (e.g. heavy metals, fossil fuels);
2. systematic increases in concentrations of substances produced by society (i.e. compounds not found in nature, like DDT and CFCs)
3. systematic degradation by physical means (e.g. deforestation, overfishing, draining aquifers);
and in that society:
4. people are not subject to conditions that systematically undermine their capacity to meet their needs.