Each year, Yale's Environmental Performance Index(EPI) ranks 180 countries based on how well they've fared at protecting human health and vulnerable ecosystems.
The EPI creates the index by giving each country a score out of 100 that's based on a number of specific metrics. The individual scores are averaged for each country to create the rankings.
The worst offending countries all share a few common traits. They're mostly impoverished, torn-apart by conflict, and have ongoing problems with drought and environmental degradation.