OSHA Allowable Levels
The requirements of OSHA prevail in the work areas "to assure so far as possible every working man and woman in the Nation safe and healthful working conditions and to preserve our human resources." OSHA regulations apply to three general areas: 1) in the laboratory, where the sulfur-asphalt mixes are designed and tested, 2) at the hot-mix plant, where the raw materials are stored and where the pavement mixes are prepared, and 3) in the haul truck and at the paver site where the pavement mix is placed.
The maximum allowable concentration of hydrogen sulfide, for prolonged exposure, is 10 ppm. The maximum allowable concentration of sulfur dioxide, for prolonged exposure, is 5 ppm. This and other applicable information on sulfur emissions is contained in table 5 for ready reference.
In general, EPA regulations start at the property line, "over-the-fence." They are reflected in the well-known State requirements for clean air control of hot-mix plants. State regulations also encompass the control of air pollution form sulfur compounds. A typical requirement limits hydrogen sulfide emissions to a net ground level concentration of 0.08 ppm averaged over any 30-minute period if the downwind concentration of hydrogen sulfide affects a property used for residential, business, or commercial purposes. Similarly, typical requirements may limit sulfur dioxide emissions to net ground level concentrations of 0.28 to 0.4 ppm.
In most cases, property line air sampling will suffice for determining the concentrations of these sulfur gases. However, provisions are also made for plant stack gas sampling and the calculation of ground level concentrations.
Instruments for Measuring Sulfur Emissions
Many instruments are suitable for measuring and monitoring sulfur gases. The following have been widely used for sulfur asphalt experimental paving projects in the United States:
High-volume-type air samplers, each assembled with motors, air suction pumps, and cages have been used to monitor sulfur dust in the ambient air. The total matter collected may be analyzed by following ASTM Standard E 30-47.
Field Measurements of Emissions
Hydrogen sulfide and sulfur dioxide emissions, monitored and measured on several SEA binder projects, have shown no occupational problems.
Typically, the concentrations of sulfur gases in the personnel areas around the hot-mix plant and at the paver have been found to be nonmeasurable or very low, from a trace to 2 ppm. A slight odor of sulfur dioxide is usually present, but it can be detected at concentrations of 0.03 ppm, far below the tolerance level of 5 ppm.
Measurements in nonpersonnel areas have been higher. In the paver hopper as new mix was being dumped, and at the paver screed level while it was being preheated, momentary levels of H2S from 1 to 20 ppm have been recorded. Such concentrations decrease rapidly with distance from the source.
Hazardous concentrations of sulfur gases have been measured in the domes of sulfur transports and asphalt transports upon arrival at the hot-plant site, in the top of poorly ventilated sulfur storage tanks containing some asphalt, and in transitory storage of SEA binders.