Below is an excerpt of the chapter on Hydrogen Sulfide from “Air Quality Guidelines for Europe, Second Edition” published by the World Health Organization, Regional Office for Europe, Copenhagen.

6.6 Hydrogen sulfide

Exposure evaluation
Typical symptoms and signs of hydrogen sulfide intoxication are most
often caused by relatively high concentrations in occupational exposures.
There are many occupations where there is a potential risk of hydrogen
sulfide intoxication and, according to the US National Institute for Occupational
Safety and Health (1), in the United States alone approximately
125 000 employees are potentially exposed to hydrogen sulfide. Low-level
concentrations can occur more or less continuously in certain industries,
such as in viscose rayon and pulp production, at oil refineries and in geothermal
energy installations.

In geothermal areas there is a risk of exposure to hydrogen sulfide for the
general population (2). The biodegradation of industrial wastes has been
reported to cause ill effects in the general population (2). An accidental
release of hydrogen sulfide into the air surrounding industrial facilities can
cause very severe effects, as at Poza Rica, Mexico, where 320 people were
hospitalized and 22 died (2). The occurrence of low-level concentrations of
hydrogen sulfide around certain industrial installations is a well known fact.

Health risk evaluation
The first noticeable effect of hydrogen sulfide at low concentrations is its
unpleasant odour. Conjunctival irritation is the next subjective symptom
and can cause so-called “gas eye” at hydrogen sulfide concentrations of 70–
140 mg/m3. Table 16 shows the established dose–effect relationships for
hydrogen sulfide.

The hazards caused by high concentrations of hydrogen sulfide are relatively
well known, but information on human exposure to very low concentrations
is scanty. Workers exposed to hydrogen sulfide concentrations
of less than 30 mg/m3 are reported to have rather diffuse neurological and
mental symptoms (4) and to show no statistically significant differences
when compared with a control group. On the other hand, changes in haem
synthesis have been reported at hydrogen sulfide concentrations of less than
7.8 mg/m3 (1.5–3 mg/m3 average) (5). It is not known whether the inhibition
is caused by the low concentrations or by the cumulative effects of
occasional peak concentrations. Most probably, at concentrations below
1.5 mg/m3 (1 ppm), even with exposure for longer periods, there are very
few detectable health hazards in the toxicological sense. The malodorous

property of hydrogen sulfide is a source of annoyance for a large proportion
of the general population at concentrations below 1.5 mg/m3, but from the
existing data it cannot be concluded whether any health effects result. The
need for epidemiological studies on possible effects of long-term, low-level
hydrogen sulfide exposure is obvious. A satisfactory biological exposure
indicator is also needed.

The full text excerpt of the chapter on Hydrogen Sulfide can be read here:  AQG2ndEd_6_6Hydrogensulfide.
The full document can be read online:  http://www.euro.who.int/__data/assets/pdf_file/0005/74732/E71922.pdf.