Late last year, the mobile phone industry passed a remarkable milestone, one that not so many years ago it didn't even expect to reach. Media sites and blogs around the world buzzed as the news was announced with equal measures of excitement, amazement and, in some cases, guarded jealousy. We'll never know who it was, or where it was, but on that day someone, somewhere bought a mobile phone and tipped global sales past the three billion mark. "More than half the world's population now own a phone" was a typical headline.

Of course, this isn't strictly true. For a start, we don't know exactly how many people live on our crowded little planet, and increasing numbers of people own more than one phone. But, nevertheless, it was a remarkable achievement for an industry still so relatively young and one that never really expected the kind of boom we're experiencing today. Adoption of these devices, which were originally the exclusive playthings of business people and the better-off, has indeed been breathtaking. To give you some idea, in the time it's taken you to read this paragraph, another 3,000 or 4,000 phones will have been sold. Nokia alone is reported to sell in the region of 17 phones per second.

When numbers become so large, the reality behind them becomes increasingly difficult to comprehend. For me, global mobile sales passed that point a long time before this recent milestone was reached, and trying to picture what anything like 3 billion of the things might look like is mission impossible. There is little doubt that mobile phones are proving incredibly empowering, and I spend a lot of my time studying their impact in the developing world. We're familiar with the stories on how mobiles will solve that "inconvenient little problem" more commonly known as the digital divide. Mobile technology has indeed revolutionised many aspects of life in the developing world, where the number of mobile connections almost universally overtook the number of fixed lines in the blink of an eye (which is why they're regularly referred to as a "leapfrogging technology"). If further evidence were needed, increasing bodies of research are pointing to mobile penetration as having a strong positive impact on GDP, and for many people, their first telephone call will be on a mobile.

Despite the many incredible things happening around the world, one thing that continues to trouble me is the environmental impact of the number of mobile phones being manufactured, consumed and, in some cases, dumped.

Let's face it: 3 billion phones represent a lot of plastic. Fortunately, recycling plans have become increasingly popular in recent years, with the nonprofit sector leading the way with a wide range of initiatives, and companies such as Fonebak making a tidy profit cleaning up after everyone's left the party.

Last year, in "Running the Numbers: An American Self-Portrait," Chris Jordan set out to examine modern American culture through what he described as the "austere lens of statistics." One of his most striking images shows just short of half-a-million mobile phones in a not-so-little pile. The picture alone is staggering, but the fact that this represents the number of mobiles ditched daily in the US is even more so.

If not handled sensibly, mobile phones have the potential to deliver negative environmental impacts -- and in some cases social ones -- almost all the way along their supply chain. The raw material alone needed to produce 3 billion phones is far from insignificant. Essential ingredients such as coltan, which can be mined by hand and is in plentiful supply in places like the Democratic Republic of Congo, has been blamed for helping fuel the civil war, increasing child labor rates, fostering illegal encroachment into national parks and the deaths of endangered gorillas. Once you have the ingredients, there is the shear amount of energy required in the manufacturing process and concerns about the working conditions in the factories. And, at the end of all that, there is the carbon emitted in shipping the finished products, usually by air.