4.3. Concentration-based estimation of exposure risk
The concentration of chemicals in each MMA route is calculated by Eq. (5) using the estimated fugitive emissions, process cross-section areas (Table 4), and wind velocity at worker level (Eq. (4)). The estimation is based on the annual probability distribution of the wind speed (Fig. 2). Subsequently, the mixture hazard quotient (Eq. (6)) is calculated for the components in the route. HQmix considers additive health effects of the chemicals.
The HQmix indicates how close the concentration estimate is to the exposure limit; the higher the HQmix value, the greater the risk. In this study, the HTP values (HTP Values, 2007) are used as the exposure limits. It is often assumed that the risk is acceptable if the concentration estimate is less than the exposure limit. For carcinogens, a safety factor of 10% is used as a guideline as discussed in Section 3.5.
Fig. 4(a) presents that the HQmix value is larger at lower wind speeds and it decreases gradually as the speed is getting higher. This is due to better dilution. The corresponding plot based on the yearly cumulative wind speed probability at working level is presented as Fig. 4(b). The HQmix curves show that the C3 route is the most harmful process to health, followed by the C2/PA and ACH. The i-C4, TBA, and C2/MP routes are clearly healthier. The C3 route is the worst option because the process exhibits large harmful material of hydrogen fluoride fugitive emissions, which contributes to significantly higher HQ value compared to the other routes. The same trend is shown by the critical wind speed analysis presented in Table 5.