The most dangerous aspect of the eruption, however, is the ash fallout, both locally and globally.
Breathe this in and it’ll lacerate your lungs and form a glassy cement. It’s also about six times denser than water, which means plenty of architecture would collapse under its weight as it accumulates on rooftops. Poland points out that “even a few tens of centimeters of wet ash could cause weak buildings to buckle.”
Roads and sewer systems would clog and break down, water supplies would be contaminated, and electrical grids would short out. Millions of homes could become uninhabitable.
In this sense, those taking shelter in Montana, Idaho, and Wyoming would be at the highest risk of harm. They would be so for up to a month, which is a fairly solid bet as to how long the eruption would ultimately be.
An area about 80 kilometers (50 miles) around the vent would be covered in 3 meters (about 10 feet) of ash in just a few days. Simulations have also shown that a supereruption could bury Salt Lake City and its surroundings beneath a meter (3.3 feet) of ash.
Assuming there’s no strongly prevailing winds, Denver would get about 30 centimeters (about a foot), whereas Calgary would get about 10 centimeters (3.9 inches). The Federal Emergency Management Agency (FEMA) would help with the cleanup/relocation for many months or even years.
Elsewhere – say San Francisco, Los Angeles, Seattle, Minneapolis, and Chicago – would receive about 3 centimeters (1.2 inches). A fine layer would make it as far as Miami, New York, and Toronto within a few days, still enough to cause vehicles to break down and water to become unpotable.
Flights would be grounded or diverted away from the United States – at least for a few weeks – and it’s almost certain that the National Guard and perhaps the military would be drafted in to help evacuate many tens of millions of people from the affected region.
The death toll is extremely difficult to predict, but Poland suggest that "if people were present in the vicinity of the eruption – say, within a few tens to perhaps a few hundred kilometers – they would be in peril."