Since catastrophic geologic events occur so rarely, it is difficult to calculate the exact risks of another event, but that is no reason to be unprepared.
Proximity to a Volcanically Active Area
There is debate among scientists what is considered "active". The last eruption of the Yellowstone caldera occurred about 70,000 years ago, while the last catastrophic eruption was 600,000 years ago. The largest eruption was approximately 2 million years ago. View a diagram to put those eruptions into context.
Although the last eruption was around 70,000 years ago, most geologists agree that the Yellowstone caldera is still active. The Yellowstone Volcano Observatory is one of 5 USGS volcano observatories in the U.S. In partnership with the University of Utah and the National Park Service, the observatory constantly monitors the status of the Yellowstone caldera for any abnormalities and continuously records geologic data. Each month they provide a report on geologic activity in the area.
Likelihood of an Eruption
With all of that said, many scientists do not know if Yellowstone's caldera will ever have another catastrophic eruption. Based on Yellowstone's past history, the possibility of another caldera-forming eruption is 0.00014%. Realize, however, that this is only based on the 3 past major eruptions, so it is hardly close to being statistically accurate.
Either way, there is currently no evidence to suggest that a catastrophic event at Yellowstone is imminent, and such events are unlikely to occur in the next few centuries. Historically there have been smaller eruptions and lava flows in Yellowstone as well, and it is possible for these to occur again. Scientists, however, have no evidence to suggest that these types of events will occur in the near future either.
Living Downstream Along the Predominant Jet Stream From Other Volcanically Active Areas
Teton County can be affected by a volcano without the eruption of the Yellowstone caldera. One of the most volcanically active areas in the United States today is the Pacific Northwest. Since Teton County lies to the east of this area (which is the predominant wind direction), we can be affected by ash and other gases as they are carried away from the eruption site.
View a diagram to see how within 9 hours the ash plume from the May 18, 1980 eruption of Mount St. Helens reached Teton County. Although we only received a dusting, there were several citizens who were taken to the hospital due to respiratory issues from the ash.
After looking at the ash distribution from the Mount St. Helens eruption, look again at another diagram to put it into context with the major eruptions of the Yellowstone caldera. Then view a map to see how the ash fall from Mount St. Helens compares to that of the prehistoric eruptions of the Yellowstone caldera.