[not-an-ostrich]

One very dangerous scenario of the Indy 500 is the number of people packed together in the infield.

[motor_racer]

The Indy 500 is an unlikely target. True, there will be at least a half million people there, but the place is MASSIVE, and it's outdoors. Not a concentrated target. Conventional explosives or chemicals would be relatively ineffective there, unless multiple simultaneous detonations were arranged. Panic would be the greatest danger in most attacks.

More likely targets are enclosed spaces, with confined air and limited exit routes.