Certainly environment can affect mental health. There is a recent study linking poor air quality to inflammation and an impact on the brain and other organs. The WHO warning is based on their definition of mental health which centers around “the ability of people to cope with the stresses of life.” They predict a major impact in poorer countries with limited resources to treat mental health issues.
It is a strange concept at first glance. We know that climate change is having a dramatic impact throughout the world — and there is an ongoing debate regarding the specifics — but the WHO stunned many when they announced that climate change is a threat to the mental health of many in the world’s population.