If you look across all the green building standards and product-certification systems, one consensus principle that repeatedly comes up is the use of products that are “sustainable.”

A sustainable product lowers pressure on the environment through the use of source materials that are renewable and/or sustainably harvested. That term “sustainably harvested” means that the product, or the product’s components, are harvested in a way that doesn’t permanently deplete the source of the material, nor poison or ruin the surrounding area, nor — in detailed analysis — pollute the air on its way to market.

That’s a fairly academic description, so let’s take a practical example. I have an iPod and I suspect you do too. A green product? You bet! By downloading songs off the Internet, I help reduce pollution, because a download avoids the production and shipping of CDs and plastic boxes and the printing of liner notes. The iPod is green in its application.

But is the product “sustainable” if the iPod is made in a filthy plant in China and powered by coal generating plants here in the United States? In other words, does the manufacture and power generation for the seemingly green iPod poison the air we all breath? In fact it does, thereby ruling the iPod out as a truly sustainable product, albeit with green properties. If you were to power the iPod with hydro or solar and ensure that the factories are well-run, you’re talking green and sustainable. See the difference?

Let’s take a remodeling example. Consider a high-performance caulk. Let’s say it’s a high-VOC product that is nasty to use because of the fumes. That caulk can be very green indeed, if it stops air infiltration, keeps out moisture, and cuts down on energy costs and mold. But is the product sustainable if the manufacturing process is poisonous? Or if the product risks the health of the contractors or the occupants who must smell it before the fumes cook off? No.