If it's present it can quickly be bad Steve. It's the 2nd most toxic substance known to man, only behind Hydrogen Cyanide. Not a lot of fatalities because it's danger is so well known in oil & gas sector.
Lots of people in oilfield country smell it as they drive down the road and say "That smells like MONEY."
I say it smells like death.
At 10ppm, your sense of smell deteriorates or acclimates (olfactory paralysis) and you may think the danger is gone.
That's where one of the dangers lies.
At 30ppm, damage to the blood/brain barrier starts.
This chart, regardless of how it may look, is not linear.
Walk up on 300-500ppm and you're likely on your way to eternity.
0.03 ppm
Can smell. Safe for 8 hours exposure
4 ppm
May cause eye irritation. Mask must be used as it damages metabolism.
10 ppm
Maximum exposure 10 minutes. Kills smell in 3 to 15 minutes. Causes GAS EYE and throat injury. Reacts violently with dental mercury amalgam fillings.
20 ppm
Exposure for more than 1 minute causes severe injury to eye nerves.
30 ppm
Loss of smell, injury to blood brain barrier through olfactory nerves
100 ppm
Respiratory paralysis in 30 to 45 minutes. Needs prompt artificial resuscitation. Will become unconscious quickly (15 minutes maximum)
200 ppm
Serious eye injury and permanent damage to eye nerves. Stings eye and throat.
300 ppm
Loses sense of reasoning and balance. Respiratory paralysis in 30 to 45 minutes
500 ppm
Asphyxia! Needs prompt artificial resuscitation. Will become unconscious in 3 to 5 minutes. Immediate artificial resuscitation is required.
700 ppm
Breathing will stop and death will result if not rescued promptly, immediate unconsciousness. Permanent brain damage may result unless rescued promptly.