[Editor's Note: GreenBiz.com is pleased to present the second in a series of excerpts from Dave Douglas's new book, "Citizen Engineer: A Handbook for Socially Responsible Engineering." Read part one: "The Big Picture on Eco-Engineering Our Environmental Impact."]

When you're behind a bus that's belching black clouds of exhaust, you can't help but think about the consequences of environmentally insensitive design. It's easy to be disgusted by what the bus is doing to air quality. It's also easy to miss the fact that the billowing fumes represent only a tiny fraction of the environmental impact of the bus. It's harder still to remember that the bus may also have major environmental benefits, despite its obvious eco shortcomings.

Let's start with the total impact. Think about the individual parts that comprise the bus. Thousands of them, globally sourced from a hundred different manufacturers and suppliers around the world, were brought together to a central location for assembly.

Think of the energy expended and the waste created in the process of obtaining the basic materials and manufacturing each of those parts. Consider the greenhouse gas (GHG) emissions of the cargo planes and freighters and delivery trucks that brought the parts through the supply chain and ultimately to the assembly plant.

Think of the fossil fuels burned by workers commuting to and from that final assembly plant -- and all of the other assembly plants where the subcomponents were built.

Consider the fact that some of those parts may have used toxic and carcinogenic substances either in the manufacturing process or within the parts themselves, and that those substances will eventually be heading to our landfills. Consider the waste materials left over after the bus has served its useful life: the massive tires with their polyester belts and steel cords, the petrochemical- based seat upholstery, the batteries with their lead oxide plates and rich assortments of acids (all of them with the potential to leach toxins into our soil and water).

And don't forget that the emissions and contaminants and waste of the bus are only one form of its environmental impact -- there's also the noise pollution, the damage done to roadways, the visual pollution of the ads and graffiti on the sides of the bus, and so on.

The bus example helps to illustrate two of our engineering challenges. First, the environmental impact of any one product or service is multifaceted and dependent on many factors, and when you're designing something to be "eco-friendly," you need to take all of those factors into consideration. Second, much of the impact of a product may lie outside any single company. If you're the owner of the bus, or even the company that did the final assembly, understanding the manufacturing process or waste issues is challenging at best.

We have no agenda against buses. In fact, they provide a great example of how something that has its own set of environmental impacts can also have a very positive effect: getting cars off the road. If we need to get 40 people from one place to another, a bus is far more environmentally friendly than 40 cars, even after the full accounting of the aforementioned impacts. This illustrates another of our challenges: Even eco-friendly programs, such as mass transit in this case, have an impact that can't be ignored. When justifying a decision based on environmental impact, it is important to consider the full lifecycles of the proposed solution and the alternatives.

This point was driven home by syndicated columnist George Will, who argued in a 2007 opinion column that a Hummer is more environmentally efficient than a Toyota Prius hybrid. He even went so far as to say that "perhaps it is environmentally responsible to buy [a Hummer] and squash a Prius with it." It's a claim that sounds patently ridiculous -- until you take a closer look at the concept of "lifecycle analysis."

Yes, the Prius is fuel-efficient by today's standards, but when you factor in the environmental costs of mining and smelting the zinc required for the battery-powered second motor, the production processes for turning the zinc into the component that goes to a battery factory in Japan, and the expected life of a Prius (109,000 miles) versus the expected life of a Hummer (207,000 miles), the environmental benefit of the Prius becomes fuzzier. (There is also a subtler argument that even if today's hybrid lifecycle isn't as favorable as we might expect, future ones are apt to be better. Why? Because the market for hybrids today fuels the R&D dollars for future engineering of what, at a fundamental level, is an innovation in engine efficiency.)

Although some of the numbers behind Will's analysis are suspect -- and the Sierra Club's attitudinal advice columnist "Mr. Green" has issued a detailed rebuttal2 -- the point is well taken: The environmental impact of any product must be measured over its full lifecycle, not just at a moment in time in its useful life.

As we noted earlier, trying to compare the relative impacts of two products or approaches can often be difficult, especially when the impacts aren't directly comparable. For example:

• Cloth versus disposable diapers: It's easy to point to the 20-year degradation time frame of plastic diapers and argue for cloth, but when you factor in the environmental cost of producing, distributing, washing and drying, using pick-up and delivery services, and ultimately disposing of cloth diapers, the debate is much less clear. What value do we place on long-term waste versus short-term energy and fresh-water usage?

• Paper versus plastic: Consumers have now been conditioned to have a knee-jerk reaction against all things plastic, but when you begin to consider the energy that's used to create a paper bag -- cutting the timber, manufacturing the paper, processing the various glues and resins needed to transform the paper into a bag, distributing it to stores, and so forth -- plastic may not look so bad.

Sometimes the best answer comes from simply looking at the problem differently. Rather than analyze all of the variables involved with the paper versus plastic discussion, some stores are now promoting a whole new option: asking shoppers to bring a couple of recyclable bags with them to the store. Or consider what Netflix did for the traditional video rental model. Rather than try to find new ways to make the old model more efficient, the company radically changed the whole distribution system and cut the carbon footprint of video rental by orders of magnitude (no more deliveries of new releases to thousands of video stores, no more back-and-forth trips by consumers to rent and return discs).


Photo CC-licensed by Flickr user target=new>The Tenth Dragon.