Filters are used in buildings to collect dust, dirt, or chemicals from air to ensure that we don't take contaminants into our bodies. Sailors must be protected in their environment as well, so engineers at a Navy research lab in Dahlgren, Virgnia, are designing and testing air filters to keep personnel safe on ships. On land, air-filter systems in houses pull air from a home, pass it through a mechanical filter, and then feed it back inside. But ships bring in outside air and blow it through three different filters, which can protect sailors against pollution and smoke --and even chemical and biological agents. "Being an engineer you can always see what it is you're affecting," says mechanical engineer Will Clark. "You're learning how to solve a problem, to make a difference, to change something."