3.6. Critical wind speed
Since fugitive emissions are diluted by the wind, a new occupational health concept, critical wind speed, can be defined. It refers to the minimum velocity of air necessary to maintain the level of chemicals in exposure limits (HQ ≤ 1) in local wind conditions. The critical wind speed may already provide an idea about the relative exposure level of the process concepts studied. The higher the calculated critical value, the higher the wind speed required to keep the chemicals below exposure limits, thus implying the greater relative exposure risk.
3.7. Intake-based risk assessment
The chemical intake is most commonly expressed as daily chemical intake amount. The intake is influenced by many variables that can be categorized into human-related (e.g. inhalation rate, body weight) and work-related (e.g. exposure time, frequency, duration) variables. Determination of accurate intake is often difficult. For example, exposure time, frequency, and duration vary among individuals. Site-specific information is the most reliable information source. Unfortunately, this is not yet available for non-existing plants, but needs to be estimated. The worker's average daily exposure is calculated based on local work duration; e.g. in Finland typical working hours in chemical industry is 1632 h per year (8 h per shift, 204 shifts per year). The working duration is assumed to be 45 years. In reality, exposure duration is shorter than working duration. However, due to data unavailability during the design stage, it is assumed that the exposure takes place all over the whole working time. It is also assumed that respiration rate is 10 m3 per day at work, which is one-half of one person's respiration rate in 1 day (Chan et al., 2006 and Dutch, 1982).
Different reference values are established for noncarcinogens and carcinogens. Reference dose (mg/kg-day) is used to estimate noncarcinogenic risk, which has the same calculation approach and result definition as the hazard quotient discussed earlier, but based on intake rather than concentration.