Perfume cannot be digitized. Strange as it may seem, science has not yet figured out the sense of smell. There are no devices like photocopiers or telephones to easily reproduce or transmit odors as with music, graphic, or literary works. Fragrances are naturally copy protected. They can only be shared physically-either by sending a vial in the mail, which is fun, or by actually getting together with others and sniffing. It's a form of communication that is ancient, personal and exciting. Ironically, it probably would not be happening on the scale it is today without the Internet.

People love pleasant smells, and the perfume industry is thriving. It's a $5 billion per year business that makes everyone run an odorous gauntlet upon entering any department store. Countless Internet websites and forums have appeared where people share opinions on the ever-increasing numbers of new scents. Yet perfume is not allowed in most hospitals. And who hasn't been assaulted by an annoying cheap scent on the bus or at the theatre?

Up until only 50 years ago, when perfumes were mostly made from natural ingredients, things were not so bad. The first time a synthetic chemical was ever used in perfume was in 1882 when the perfumer Houbigant put man-made coumarin in Fougère Royale. Before that, the vanilla sweet herbaceous aroma of coumarin came from natural sources, primarily the tonka bean, which had to be collected in Central American jungles, extracted and then purified before it could be used. Its scarcity made it relatively expensive. Synthetic coumarin derived from petroleum, however, is a hundred times cheaper, so manufacturers could afford to put it into many more products.

The same is true for the thousands of modern aroma chemicals, which began appearing in the 1960s and exploded onto the market along with other synthetics such as plastics. These artificial compounds can be thousands of times stronger than their natural counterparts and they are cheap, so they make up the bulk of the ingredients in commercial perfumes. In fact, today, the actual chemicals in a perfume are so inexpensive and powerful, they represent only 3% of the total cost of the product. The other 97% goes into marketing, because people always want something fresh and new-a special fragrance that just says "me". The industry is happy to provide an endless supply of such prestige brands, like Justin Bieber's Someday, a new fruity bubble-gum synthetic for pre-teen girls.

It's that animal allure of scent that is at once enigmatic, mysterious and personal. Smell is the most ancient of our senses. It recognizes small chemicals in the environment that signal everything from danger to delight, and all living things, including plants and even the smallest bacteria have some form of it. Stuart Firestein (watch video), an olfaction scientist at Columbia University in New York, says, "In biology, because things happen over evolutionary time, the more primitive something is, the longer it has had to evolve and become better, or perfected." Our sense of smell is so important that 2% of the genes in the human genome are devoted to making smell receptors. "That's like 1 in every 50 of your genes," says Firestein. For animals like rats and dogs with even more acute senses of smell, the number of genes is closer to 5% of their genomes.

Yet smell remains a mystery. While scientists have located the odor receptors in our nasal cavities, and they know how they are connected to the brain-by nerves that go directly to the oldest and deepest parts-they still do not have a satisfactory theory that explains how smell works. It has something to do with the unique shape or vibration of molecules. Exactly how that defines their smell remains unknown. At least 10,000 odorous molecules have been identified and catalogued, but a hundred thousand more are out there, and new ones are always being discovered. The most powerful often find their way into new perfumes, causing some people to become sensitized or allergic. That's why strong scents are now banned in hospitals and other places such as theatres.

For people who would prefer a more subtle approach to scent, the Internet has an answer. Now anyone can make their own personal fragrance the old fashioned way: from natural essential oils, tinctures and extracts, which are typically not as powerful as commercial synthetic formulations. And they don't persist as long in the environment. While a few natural perfumers have always been around, the Internet has caused an explosion in amateur do-it-yourself perfumery. Countless websites sell essences and other raw materials for a few dollars each, shipping them quickly by mail. Dozens of online forums and blogs contain all the information a newbie needs to get started. Online clubs offer advice, swap meets, reviews and guidance from seasoned mentors. A favorite is hosted from London by Grant Osborne. Many self-taught niche-market perfumers such as Mandy Aftel in Berkeley, Anya McCoy in Miami, and Ayala Moriel in Vancouver are making a living producing one-of-a-kind blockbuster scents at $100 for tiny 15ml bottles-all easily available online.

So while perfumery appears to be the last art form to succumb to the digital revolution, nevertheless it's enjoying a grass roots renaissance, thanks to the Internet.