About the Green Rating
The Green Rating rates just the vehicle, not how you use it
In interpreting the Green Ratings, keep in mind that they are intended strictly for vehicle-to-vehicle comparisons. For example, a vehicle's Green Rating does not improve if you carry four passengers in it instead of just yourself, even though the per-person environmental impact will clearly decrease. Similarly, the Green Rating doesn't go down if you drive aggressively, even though the environmental damage due to your driving goes up.
To enable fair comparisons among vehicles regardless of how they are used, the Green Ratings are based on uniform assumptions about how far the vehicle is driven over its lifetime and good maintenance practices. Your own effect on the planet when using a car, of course, depends on how you drive, how well you maintain the vehicle, and how many miles you drive it.
Unless a vehicle is designed for exclusive use on a particular fuel (such as a car that runs only on natural gas, like the Honda Civic GX), its Green Rating is based on commonly available motor fuel (gasoline or diesel). While Green Ratings are designed for fair comparisons among vehicles in the showroom, you might reduce your own vehicle's pollution by using an alternative fuel. For example, a flexible-fuel vehicle able to run on E85 (a blend of 85% ethanol and 15% gasoline) can have lower greenhouse gas emissions when using E85 rather than gasoline. Similarly, biodiesel can emit less global warming pollution than regular diesel produced from petroleum.
Because most of today's biofuels were not originally developed to solve global warming, their greenhouse gas reductions vary. Farmers and refiners are working on new ways to produce cleaner fuels that will provide greater climate-protection benefits as time goes on. Consumers should stay tuned.
More information on the pollutants factored into the Green Ratings
Greenhouse gases include carbon dioxide (CO2) and other emissions from the production and use of fossil fuels as well as deforestation. These global warming pollutants are accumulating in the atmosphere and causing the Earth to heat up at an unprecedented rate. Global warming brings many dangers and its consequences are already being felt. Glaciers and mountain snow packs are melting. Sea ice is thinning. Many communities will experience more sweltering days. Hazards from extreme weather events, such as intense hurricanes, are rising -- as is sea level, which heightens the impact of coastal storms and risks human tragedies and economic ruin for the many population centers located near a shoreline. Other dangers of global warming include spread of tropical diseases, destruction of habitat and disruptions to forestry and agriculture, bleaching of coral reefs, and adverse impacts on wildlife and fisheries.
Also called particulate matter (PM) and sometimes "soot," fine particles from car and truck fuel combustion are generally invisible. Many are so small they are labeled "ultra-fine." Fine particles are an especially deadly form of air pollution that can affect breathing, aggravate existing respiratory and cardiovascular disease, weaken the body's immune defenses, and contribute to cancer and premature death. Fine particles are especially hazardous for children, the elderly, asthmatics and people with cardiovascular or chronic obstructive pulmonary disease.
Nitrogen oxides (NOx) are harmful in many ways. In addition to being a direct irritant, NOx are key contributors to ozone smog, which damages lung tissue, worsens asthma, and recently has been found to increase the risk of premature death. Smog alerts bring health risks not only for people with asthma and other conditions that make them sensitive to air pollution, but even for normally healthy adults and children who can experience lung inflammation and shortness of breath if they are active in smoggy air. NOx contributes to fine particle pollution, with its dangers as noted above, and is also a cause of acid rain.
There are many kinds of hydrocarbons (HC), and the hazardous ones emitted by cars fall into categories known as "volatile organic compounds" or "reactive organic gases." These pollutants are another key contributor to ozone smog, bringing all of the health dangers noted above for NOx. Some forms of HC directly irritate the lungs; many contribute to fine particle pollution and some hydrocarbons are also air toxics, including compounds having known cancer risks.
Carbon monoxide (CO) is a poisonous gas that is deadly in high concentrations, notorious for sometimes killing people who leave a vehicle running in a closed garage, for example. CO is colorless and odorless, and when inhaled, it impairs the blood's ability to supply oxygen to the body. Even short of fatal exposures, exposure to too much CO can cause physical, mental and visual impairment and presents grave risks to individuals who have cardiovascular disease.