The EKC hypothesis suggests that as countries develop economically, pollution initially increases but then decreases after a certain income threshold is reached, while the PH hypothesis argues pollution increases with economic growth. McCollough et al (2016) find that while the US satisfies the PH hypothesis by showing pollution increasing with growth over the long run based on their analysis in Tables 1-4, it does not satisfy the EKC hypothesis.

1. What is the EKC hypothesis ? What are its implications ? What is the
PH hypothesis ? Does the US satisfy the EKC hypothesis and/or the
PH hypothesis ? What can we conclude from McCollough et al
(2016) ? How do the researchers establish this result in their Tables
1 4 ?