Concentration-time extrapolation of short-term inhalation exposure levels: dimethyl sulfide, a case study using a chemical-specific toxic load exponent.

OBJECTIVE: Dimethyl sulfide (DMS, CAS 75-18-3) is an industrial chemical. It is both an irritant and neurotoxicant that may be life-threatening because of accidental release. The effects of DMS on public health and associated public health response depend on the exposure concentration and duration. However, currently, public health advisory information exists for only a 1 h exposure duration, developed by the American Industrial Hygiene Association (AIHA). In the present work, the AIHA-reviewed data were computationally extrapolated to other common short-term durations. METHODS: The extrapolation was carried out using the toxic load equation, Cn × t = TL, where C and t are exposure concentration and duration, TL is toxic load, and n is a chemical-specific toxic load exponent derived in the present work using probit meta-analysis. The developed threshold levels were vetted against the AIHA database of clinical and animal health effects induced by DMS. RESULTS: Tier-1 levels were derived based on human exposures that resulted in an easily detectable odor, because DMS is known to have a disagreeable odor that may cause nausea. Tier-2 levels were derived from the lower 95% confidence bounds on a benchmark concentration that caused 10% incidence (BMCL10) of coma in rats during a 15 min inhalation exposure to DMS. Tier-3 levels were based on a BMCL05 for mortality in rats. CONCLUSION: Emergency responders and health assessors may consider these computationally derived threshold levels as a supplement to traditional chemical risk assessment procedures in instances where AIHA developed public health advisory levels do not exist.

On-line monitoring of gas-phase bioreactors for biogas treatment: hydrogen sulfide and sulfide analysis by automated flow systems.

Biogas is produced by biological processes under anaerobic conditions and may contain up to 20,000 ppm(v) hydrogen sulfide (H(2)S), a corrosive substance that attacks power engines and can affect the health of the industrial staff. H(2)S must be removed from the biogas, especially in co-generation facilities where the biogas is burnt for energy production. Nowadays, biofiltration is being studied and considered as an interesting alternative for removing H(2)S from the biogas besides classical chemical processes. The novelty of this work is the design and construction of an automated H(2)S on-line analyser to assess the composition of the liquid and gas phases of gas-phase bioreactors. The analyser is made of two parallel flow configurations which share the same detection device. The first configuration is a single-channel flow injection analyser (FIA) to detect S(2-) in the liquid phase. The second configuration is a continuous flow analyser (CFA) with a gaseous diffusion step (GD-CFA) for detecting H(2)S in the gas phase. The diffusion step enables separation of the H(2)S((g)) from the sample and its conversion into a detectable chemical species (S(2-)). S(2-) detection was performed with an Ag(2)S ion-selective electrode (ISE) selective to S(2-)(aq). The main response parameters of the FIA system are a linear range between 3 x 10(-5) and 1 x 10(-1) mol L(-1) S(2-) (0.61-3,200 mg L(-1)), with a sensitivity of 27.9 mV decade(-1) and a detection limit of 1.93 x 10(-5) mol L(-1) S(2-). The GD-CFA configuration presents a linear range between 400 and 10,000 ppm(v) H(2)S((g)) with a sensitivity of 26.1 mV decade(-1) and a detection limit of 245 ppm(v) H(2)S. The proposed analyser was used by analysing real gas and liquid samples with optimal results at a full-scale biotrickling filter for biogas treatment at a municipal wastewater treatment plant.

Accuracy of stone casts obtained by different impression materials.

Several impression materials are available in the Brazilian marketplace to be used in oral rehabilitation. The aim of this study was to compare the accuracy of different impression materials used for fixed partial dentures following the manufacturers' instructions. A master model representing a partially edentulous mandibular right hemi-arch segment whose teeth were prepared to receive full crowns was used. Custom trays were prepared with auto-polymerizing acrylic resin and impressions were performed with a dental surveyor, standardizing the path of insertion and removal of the tray. Alginate and elastomeric materials were used and stone casts were obtained after the impressions. For the silicones, impression techniques were also compared. To determine the impression materials' accuracy, digital photographs of the master model and of the stone casts were taken and the discrepancies between them were measured. The data were subjected to analysis of variance and Duncan's complementary test. Polyether and addition silicone following the single-phase technique were statistically different from alginate, condensation silicone and addition silicone following the double-mix technique (p < or = .05), presenting smaller discrepancies. However, condensation silicone was similar (p > or = .05) to alginate and addition silicone following the double-mix technique, but different from polysulfide. The results led to the conclusion that different impression materials and techniques influenced the stone casts' accuracy in a way that polyether, polysulfide and addition silicone following the single-phase technique were more accurate than the other materials.

Accuracy of stone casts obtained by different impression materials

Several impression materials are available in the Brazilian marketplace to be used in oral rehabilitation. The aim of this study was to compare the accuracy of different impression materials used for fixed partial dentures following the manufacturers' instructions. A master model representing a partially edentulous mandibular right hemi-arch segment whose teeth were prepared to receive full crowns was used. Custom trays were prepared with auto-polymerizing acrylic resin and impressions were performed with a dental surveyor, standardizing the path of insertion and removal of the tray. Alginate and elastomeric materials were used and stone casts were obtained after the impressions. For the silicones, impression techniques were also compared. To determine the impression materials' accuracy, digital photographs of the master model and of the stone casts were taken and the discrepancies between them were measured. The data were subjected to analysis of variance and Duncan's complementary test. Polyether and addition silicone following the single-phase technique were statistically different from alginate, condensation silicone and addition silicone following the double-mix technique (p < .05), presenting smaller discrepancies. However, condensation silicone was similar (p > .05) to alginate and addition silicone following the double-mix technique, but different from polysulfide. The results led to the conclusion that different impression materials and techniques influenced the stone casts' accuracy in a way that polyether, polysulfide and addition silicone following the single-phase technique were more accurate than the other materials.

Proportioning effect on physical and chemical properties of polysulfide impression material.

This study investigated the effect of variation in proportioning polysulfide base and catalyst. Light, regular, and heavy-bodied pastes were mixed in linear equivalents of base-to-catalyst proportions of 30/70, 40/60, 50/50, 60/40, and 70/30. Compression set, strain in compression, and consistency tests conformed to the American Dental Association (ADA) Specification 19 guidelines. The amount of solventleachable components and differences in the cross-linked nature of the cured products were studies. Compression set and strain in compression tests met the ADA specification guidelines. Some consistency values fell outside of the ADA specification limits. The leachable content increased with increasing proportions of catalyst, but polymer cross-linking was remarkably similar for all bodied materials tested. Clinicians need not fear a significant property alteration in polysulfide impression material by the differences (within +/- 20% of the manufacturer's suggested lengths) in proportioning the component pastes with "eyeball" estimation as directed in the instructions.