For those who can't find enough time in a day to worry about all of life's possible dangers, there's a new book to help them prioritize. Risk: A Practical Guide for Deciding What's Really Safe and What's Really Dangerous in the World Around You is a fascinating assessment of the level of threat posed by various illnesses, accidents, environmental pollutants and other factors. David Ropeik, director of risk communication at the Harvard Center for Risk Analysis, and his colleague George Gray, a toxicologist, evaluate such real or perceived menaces as cell phones, biological weapons, pesticides, mad cow disease and medical errors. For each entry, they analyze the potential hazards and offer tips for reducing risk. They also include a ""Risk Meter""-a chart that shows likelihood of exposure and severity of consequences at a glance. 25 b&w illus.