Occupational Health and Safety Act, 1993 (Act No. 85 of 1993)RegulationsRegulations for Hazardous Chemical Agents, 2020AnnexuresAnnexure 3 : Hazardous Chemical Agent GuidelinesGuidance on medical surveillance and biological monitoringApplying occupational exposure limitsLong-term and short-term exposure limits |
| 58. | Effects of exposure to agents hazardous to health vary considerably depending on the nature of the agent and the pattern of exposure. Some effects require prolonged or accumulated exposure. The long-term (eight-hour TWA) exposure limit is intended to control such effects by restricting the total intake by inhalation over one or more work shifts, depending on the length of the shift. Other effects may be seen after brief exposures. Short-term exposure limits (usually 15 minutes) may be applied to control these effects. For those HCAs for which no short-term limit is specified, it is recommended that a figure of three times the long-term limit be used as a guideline for controlling short-term peaks in exposure. Some workplace activities give rise to frequent short periods (less than 15 minutes) of high exposure which, if averaged over time, do not exceed either an eight-hour TWA or a 15-minute TWA. Such exposures have the potential to cause harm and should be subject to reasonably practicable measures to protect the worker. |
| 59. | Ceiling limits are set for HCAs that are predominantly fast acting and whose OELs are more appropriately based on this particular response. HCAs with this type of response are best controlled by an OEL-C that should not be exceeded. It is implicit that the manner of sampling to determine non compliance with the OEL-C for each similar exposure group must differ. Consequently, a single, brief sample that is applicable to an OEL-C is not appropriate to the OEL-TWA; here a sufficient number of samples are needed to permit determination of a TWA concentration throughout a complete cycle of operation or throughout the work shift. Whereas the OEL-C places a definite boundary that exposure concentrations should not be permitted to exceed, the OEL-TWA requires an explicit limit to the excursions which are acceptable to the promulgated TLV-TWAs. HCAs with ceiling limits are identified in Table 2 and 3 in Annexure 2, in the column "STEL/C", by means of a "C" notation. |
| 60. | Both the long-term and short-term exposure limits are expressed as airborne concentrations averaged over a specified period of time. The period for the long-term limit is normally eight hours, when a different period is used this is stated. The averaging period for the short-term exposure limit is normally 15 minutes, such a limit applying to any 15-minute period throughout the working shift. Exposure to agents hazardous to health should be calculated according to the approved method, which is reproduced in Annexure 3. |