Among these, only ground-level ozone exceeds the federal health standard in the Houston region. They make no significant contribution to regional NOx. Further, the VOCs coming from trees and other vegetation are not harmful to health, as are many of manmade VOCs. So, the conclusion of the first section of the report is that, yes, trees have emissions but 'emission' and 'pollution' are not synonymous terms. Embedded in the word 'pollution' is the concept of harm.
Trees cool the air.
Cooler air temperatures mean lower ozone levels, reduced demand for air conditioning, and reduced pollution from power generation. Trees are excellent buffers against highway and industrial pollution. Urban parks, tree-lined streets, and greenbelts are also highly effective visual and noise barriers. Trees reduce erosion, runoff, and water pollution.