This Friday will be the first National Work at Home Day, aimed at encouraging employees to give telecommuting a try, for the sake of the planet and their own productivity. About 19.6 million Americans telecommuted to work in 1999, up from 4 million in 1990, and that number will continue to grow as technology and the Internet make it easier for many people to do their work from their own domiciles. Fewer folks trekking to offices means fewer cars clogging the roads and polluting the air. And word is that people who work at home, even just one day a week, are often more productive.