Volcano safety is, to put it mildly, a controversial subject; see the article on Volcanoes and, particularly, its discussion page for some of the issues. Compared to many other active volcanoes, Mount St. Helens has been studied extensively, and therefore has a relatively well-defined "safety envelope" that allows informed decision making as regards trail closures, etc. Even St. Helens, however, is prone to bouts of unexpectedly violent behavior, as for example on 8 March 2005 when an explosive event sent ash and steam to elevations above 35,000 feet 10 km essentially without warning. The monument, therefore, has established a policy regarding road and trail closures that at first glance appears unnecessarily conservative -- but it is not. Believe it. The closures aren't there simply to inconvenience and irritate you. If a trail is closed due to eruptive hazard, stay off the trail.
Other than the volcanic activity, St. Helens poses basically the usual set of hazards associated with mountainous country -- changeable weather, potential for road closures due to snow in the winter, etc. One extra thing to be aware of is that much of the area on the north side of the mountain, particularly the northeast, does not yet have many travel services, even things as basic as gas stations. When leaving the main roads to head for the observatory, or particularly the Windy Ridge viewpoint and trailhead, it's wise to have a full gas tank.
fees/permits
Monument passes are sold for single-day admission to the visitor centers along Washington 504.
16 years or older: US $8
15 years or below: Free.
Golden Passports are honored at Mount St. Helens.
The Golden Eagle Passport US$65 annually is good for admission to any U.S. Forest Service or National Park Service site that charges admission fees.
The Golden Age Passport US$10 lifetime provides individuals over 62 years of age all Golden Eagle benefits plus a 50% discount at campgrounds, boat launches, and specialized interpretive services.
The Golden Access Passport free provides all Golden Age benefits to individuals with documented disabilities.
Golden Passports are available only to U.S. citizens or permanent residents online at (http://www.natlforests.org), or at any U.S. Forest Service or National Park Service office.