Published by the Massachusetts Department of Environmental Protection (http://www.mass.gov/dep/recycle)

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Hydrogen Sulfide and Sulfides:
Sulfides are naturally occurring gasses that often give a landfill gas mixture its rotten egg smell. Sulfides
can cause unpleasant odors even at very low concentrations. Hydrogen sulfide is a colorless, flammable
gas and is one of the most common sulfides responsible for landfill odors. Some people can smell
hydrogen sulfide (individual’s odor threshold) at concentrations as low as 0.5 parts per billion (ppb).
However, the odor threshold can vary significantly among individuals based on the olfactory sensitivity of
the person. For many compounds, including hydrogen sulfide, there is a wide variability in published odor
thresholds (refer to Table 1). Odors alone cannot be relied upon as providing an early warning for
elevated concentrations of hydrogen sulfide. “At concentrations around 100 ppm,” (parts per million)
“no odor is detected due to a loss of olfactory sensation, resulting in loss of warning properties at lethal
levels.” (Integrated Risk Information System (IRIS)). Hydrogen sulfide is more dense than air, and
therefore, more likely to pool at lower elevations under still conditions, depending upon topography.

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Above paragraph is an excerpt from  Appendix A – Basics of Landfill Gas (Methane, Carbon Dioxide, Hydrogen Sulfide and Sulfides):  http://www.mass.gov/dep/recycle/laws/lfgasapp.pdf 

The full document is Control of Odorous Gas at Massachusetts Landfills (http://www.mass.gov/dep/recycle/laws/lfgaspol.pdf) published on the website of the

COMMONWEALTH OF MASSACHUSETTS
EXECUTIVE OFFICE OF ENVIRONMENTAL AFFAIRS
DEPARTMENT OF ENVIRONMENTAL PROTECTION