Laurilu, the smell is hydrogen sulfide. Methane, btw, is odorless.
These types of stories are not unusual:
The National Institute for Occupational Safety and Health (NIOSH)
"Hog Farm Co-Owner and Employee Die of Hydrogen Sulfide Poisoning in Manure Pit--Minnesota"
A 27-year-old male hog farm employee (victim #1) died as a result of hydrogen sulfide poisoning when he entered a manure- waste pit to extract a pump. The 46-year-old farm co-owner, the victim’s uncle (victim #2), also died from hydrogen sulfide poisoning when he entered the pit in a rescue attempt.
That involves very high acute levels, and seizures do occur before death. You'd want to know about chronic lower levels. There's more to talk about, if you reply.
Methane causes harm by displacing oxygen, something like carbon monoxide.