"A number of years ago, Enrique Penalosa, 
the former mayor of Bogotá, Colombia, 
observed that while we know a great deal 
about what constitutes healthy habitat for 
a whale or a mountain gorilla, we are far 
less clear about what constitutes a healthy 
habitat for human beings. In recent 
years, however, scientists worldwide have 
made major strides in identifying at least 
one key component in a healthy human 
habitat: green."