III. H2S Health Hazard Assessment for Permitting and Regulation of Wells, Facilities and Pipelines
July 22, 1997 | By Arlin Wasserman
and James Skifstad
Great Lakes Bulletin News Service
Hydrogen sulfide is a highly toxic substance which may be either in free gaseous form or dissolved in liquid water, in liquid hydrocarbon oils, or in specialized chemical solutions utilized for "sweetening" purposes in some facilities under ordinary conditions. At various concentrations, hydrogen sulfide is often present in gas mixtures and dissolved in hydrocarbon oils or in aqueous solutions, any of which may emerge from oil or gas wells.
The presence of wells, flowlines, facilities, and gathering lines containing gases and liquids with dangerous levels of H2S in the proximity of the public presents a serious health hazard in the event of accidental releases. The state has a clear legal obligation to avoid the hazards to public health and safety such locations would present. No member of the public should be required to accept the threat of exposure to dangerous levels of H2S as a result of state permitting or regulating activities.
The evaluation of health hazards to the general public should become an integral part of the siting, permitting and regulating functions served by DEQ and MPSC with respect to the oil and gas industry in Michigan. To do so in a credible and responsible fashion, the assessment of health hazards should be based on the best of medical and engineering technology.
The health profession can supply information regarding the nature of the health threat to an individual when exposed to a particular concentration of H2S in the atmosphere in a given form for a specific period of time.
*For gaseous H2S there exists an upper limit for the concentration (10 ppm) acceptable for a healthy male laborer, over a normal 8-hour work period, according to the ubiquitous federal OSHA standard.
*Extending this to the general public, to account for infants, women, the elderly, those with respiratory ailments or chronic ailments of other types, Jim Bedford of the Department of Community Health sets a guideline public exposure limit at 0.1 ppm.
*Other states have set recommendations for public health that are below 0.1 ppm. There is evidently no such extension to maximum exposure limits for public health purposes by the federal government.
The nature of the exposure of the public to accidental releases of H2S gas can occur in a variety of ways, usually exhibiting varying concentrations over time. While a time average concentration (values often determined in atmospheric dispersion analyses) might be low, there may still be spikes in the concentration that could kill a person. Care is evidently needed in utilizing data for a fixed concentration level in making health hazard assessments.
Recent findings seem to show significant health problems are likely even for chronic low-level exposures to H2S gas. It is important to recognize that almost any single concentration level serving as a measure of the health hazard may not be a credible measure of the true situation unless the actual exposures are reasonably uniform in time.
Additional research into the character of health effects arising from distinct types of exposure (high concentrations for short periods, etc.) and the continued refinement of dispersion models, will eventually help to resolve such issues.
Of course, the effects also depend on the health, age, and physical condition of the individual, introducing other factors into the evaluation. It may happen that a fixed concentration of H2S is not as relevant as information about the nature of a transient exposure to the actual health hazard.
There also are other physical exposure mechanisms. When liquid fogs or mists containing H2S in solution comprise the exposure, as often arises in accidental releases, the physical nature of the exposure is distinct from that of a gas mixture. Little has been done to study the health hazards that type of exposure entails, despite the near-ubiquitous presence of such fogs around accidental releases, and their tendency to persist for longer durations and distances from the release site.
Further research into the character of the toxicity of H2S may be expected to lead to more comprehensive measures for the health hazards that various types of exposure present. That could still result in a single value for some parameter incorporating relevant physical variables, though it need not be in the form of a single concentration level for H2S in the gases present.
Atmospheric Dispersion Models
An atmospheric dispersion model is a mathematical description of the turbulent mixing of streams of fluid (gases and liquids) emerging from an accident site into the atmosphere, subject to winds, atmospheric temperature, pressure, and humidity variations, stability conditions, etc.
It must be understood that even the simplest models for atmospheric dispersion analyses-- those for pure gas releases -- are approximate, at best.
*Turbulence and turbulent mixing remain current research topics in engineering.
*Most models sanctioned by EPAdo not draw on the most advanced available modeling of the turbulence.
*The results sought in such studies normally do not warrant such detailed, computer-intensive efforts.
*More often, the models involve simplifications, wherein details in the representation of the physics are only approximate.
*Those models are then usually further adjusted to come into agreement with selected experimental studies to provide reasonable results for some variables, such as concentrations, over some ranges of conditions. (continued on next page)