Astronauts in the ISS are only exposed to about 50 to 2,000 msv during their time in space. To cause immediate hair loss and blood vomiting the level would have to be about 10,000 msv with death being almost inevitable within weeks. If exposure causes victims to fall asleep aka coma, such as what Garou did and if we low-balled the S-class being in Garou's presence to be over 3 minutes from the time he arrived, they would've been exposed to about 60,000 to 70,000 msv. Even being exposed to the Chernobyl reactor core for 10 minutes after the meltdown only reaches 50,000 msv. If the S-class were in Garou's presence between 2 min to a couple of seconds then exposure would be about 80,000 to 100,000 msv.
Keep in mind time and amount of exposure factor into these numbers. Like being exposed to 2,000 msv over an hour is worse than being exposed to 2,000 msv over a year.