NIOSH HHEs supplied a majority of the measurements (). More than 90% of the reported levels were individual measurements and <30% were censored at the LOD. A majority of reported levels for methylene chloride and 1,1,1-trichloroethane were from long-term personal measurements, but trichloroethylene reported levels were split between personal (47%) and area (53%) measurements. The median measurement year for all three solvents was the early 1980s.
Evaporation was the most frequent primary mechanism of release for 1,1,1-trichloroethane and trichloroethylene and the most frequent secondary mechanism of release for methylene chloride (). Evaporation was specified as one of the two mechanisms of release for 87% of the database. Most reported levels were associated with open/both process conditions, indoor locations, and not confined spaces. Distributions for the other exposure determinants varied by solvent.
Exposure determinant distributions
Most determinants were abstracted from the literature (). Process temperature (for methylene chloride and 1,1,1-trichloroethane) and usage rate and confined space (for all three solvents) had ~50% or more of determinant values derived from judgment and so were excluded from the modeling process.
Distribution of decision values for each determinant
In the regression models, parameter estimates for measurement year were negative and highly statistically significant, resulting in estimates of declines of 2.7, 3.5, and 6.7% per year for methylene chloride, 1,1,1-trichloroethane, and trichloroethylene, respectively (). Estimates for measurement duration and type were not consistent. For example, compared to long-term personal measurements, short-term personal measurements were significantly lower for methylene chloride but significantly higher for 1,1,1-trichloroethane. Measurement duration, type, and year explained a fair amount of the variability with R-squared values of 22, 13, and 42% for methylene chloride, 1,1,1-trichloroethane, and trichloroethylene, respectively.
Parameter estimates and standard errors (SE) for models of natural log-transformed chlorinated solvent levels (p.p.m.)
Modeled parameter estimates for the three solvents were generally as expected. Because of small numbers and to increase interpretability, the category of active mechanism of release was combined with aerosolization for methylene chloride and with evaporation for trichloroethylene. For methylene chloride, compared to active/aerosolized, primary evaporation was associated with a 50% decrease and secondary evaporation with a 70% decrease. For 1,1,1-trichloroethane, compared to evaporation, active primary mechanism was associated with a 7-fold increase and aerosolized primary mechanism with a 30-fold increase; active and aerosolized secondary mechanisms were associated with 4-fold and 5-fold increases, respectively. For trichloroethylene, primary aerosolized was associated with a 2-fold increase and secondary aerosolized with a 10-fold increase compared to evaporation/active. Effective LEV was associated with 60–70% lower levels, and for trichloroethylene, ineffective LEV was associated with 30% lower levels, compared to no LEV. IMD was associated with 50% lower levels for methylene chloride and 1,1,1-trichloroethane, but only 20% lower levels for trichloroethylene. Elevated process temperature was associated with a 4-fold increase for trichloroethylene. Compared to working indoors, working at an outdoor location was associated with 90–95% lower levels for methylene chloride and 1,1,1-trichloroethane. Working in close proximity to the source was associated with an ~3-fold increase. The proportion of variation in the reported levels explained by these models was 36, 38, and 54 for methylene chloride, 1,1,1-trichloroethane, and trichloroethylene, respectively.
Standardized residuals for the reduced models were approximately normally distributed; however, formal statistical tests (Shapiro–Wilk) rejected the null hypothesis of normality for methylene chloride (P
0.0026) and trichloroethylene (P
0.034) (data not shown). A visual inspection (box plots) of the standardized residuals indicated no major problems with heteroscedasticity.
Models described in were used to predict AM exposure intensity levels for the exposure scenarios (described above). Predicted AM exposure intensity levels for the evaluated exposure scenarios ranged from 0.051 to 160 p.p.m. (median 2.8 p.p.m.) for methylene chloride, from 0.0013 to 200 p.p.m. (median 0.67 p.p.m.) for 1,1,1-trichloroethane, and from 0.21 to 3700 p.p.m. (median 30 p.p.m.) for trichloroethylene (). The percent of predicted exposure levels exceeding current ACGIH TLVs was comparable to the percentage of reported levels exceeding the TLVs for 1,1,1-trichloroethane (0 and 2.1%, respectively), but lower for methylene chloride (4.7 and 23%, respectively) and higher for trichloroethylene (71 and 45%, respectively). No predicted exposure intensities for the 192 evaluated methylene chloride exposure scenarios or the 432 evaluated 1,1,1-trichloroethane scenarios exceeded the 1% SVP threshold. For trichloroethylene, the large difference in percentage of reported versus predicted levels exceeding the current ACGIH TLV and the higher percentage of predictions above the 1% SVP threshold were due predominantly to estimates derived from the earlier decades (i.e. 1950s–1970s).
Percent of measurement data and predicted intensities (for the evaluated exposure determinant scenarios) exceeding various thresholdsa
Predicted exposure intensity levels were generally consistent with the measurement data, with the median and mean reported levels from the measurement database increasing with the predicted intensity scores (). For example, estimated methylene chloride exposure intensities for 85 (44%) of 192 exposure determinant scenarios were assigned to the lowest score (<2 p.p.m.). Air measurements in the database were available for six scenarios, representing 20 reported levels. The median and mean of these measurements were 1.8 and 6.5 p.p.m., respectively.
Internal consistency of predicted intensities
Mean Spearman correlation coefficients between the reported levels and the predicted intensities in the 20% validation samples were 0.21 (95% CI: 0.09–0.32), 0.47 (95% CI: 0.36–0.57), and 0.61 (95% CI: 0.49–0.72) for methylene chloride, 1,1,1-trichloroethane, and trichloroethylene, respectively.