Like lawnmowers, most gasoline-powered leaf blowers run on a lightweight, compact two-stroke engine. During operation, about 30% of the fuel the engine uses fails to undergo complete combustion. As a result, the engine emits relatively large quantities of carbon monoxide, nitrous oxides and hydrocarbons into the atmosphere. This Washington Post article from 2013 explains the problem in greater detail: https://www.washingtonpost.com/national/health-science/how-bad-for-the-environment-are-gas-powered-leaf-blowers/2013/09/16/8eed7b9a-18bb-11e3-a628-7e6dde8f889d_story.html
AARP recommends raking leaves as a way to keep fit and burn calories while enjoying the outdoors: http://www.aarp.org/health/fitness/info-02-2009/raking_leaves.html. Consumer Reports has tested the efficacy of leaf blowers compared with raking and recommends an ergonomic rake for getting the job done, avoiding injury, and protecting the environment: http://www.consumerreports.org/cro/news/2011/09/should-you-buy-a-leaf-blower-or-a-rake/index.htm
More and more gardeners are wondering whether it’s even necessary to remove leaves from the garden and lawn. A mower can turn fallen leaves into valuable mulch, but its gasoline-powered engine may be just as polluting as the leaf blower’s. Raking leaves and adding them to your compost bin, or simply raking them into garden beds, is probably the most environmentally-friendly option of all: http://www.treehugger.com/lawn-garden/skip-rake-and-leave-leaves-healthier-greener-yard.html.
Photos from Flickr by Shelton Dunning (rake) and Marco Verch (leaf blower)