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Now 6.2 BACKGROUND READINGS All direct-reading instrument readings will be evaluated relative to background readings, not "meter zero". Prior to the start of work at each shift, and whenever there is a significant shift in wind direction, instrument readings will be obtained upwind of the site work zone in order to determine the level of "background" readings from local vehicle traffic, emissions from nearby operations unrelated to the site, etc. Site readings will be evaluated against these background readings (i.e. , if an action level is listed as 20 ppm, it is evaluated as 20 ppm above background) . 6.3 DATA LOGGING All exposure monitoring data, including background readings, will be logged r+ in the field log book. The results of daily instrument calibrations can either be logged on the form provided in the Attachments or in the field log book. All monitoring instruments will be calibrated, in accordance with the manufacturer's instructions or the guidance found in the Attachments, prior to the start of each shift. Calibration should also be performed when inconsistent or erratic readings are obtained. If an instrument cannot be calibrated to specification, or becomes otherwise inoperable, all invasive site work (i.e. , drilling, excavating) will cease until the instrument is appropriately repaired or replaced; the PM, DHSM or HSP Preparer should be contacted for further guidance. 6.4 DUST CONTROL If drilling operations generate sustained visible dust, a water mist will be applied to reduce dust generation. If the mist is not effective in reducing dust generation, personnel will don respirators (half-face or full-face as appropriate for analyzer readings) with combination organic vapor-HEPA cartridges (such as MSA's GMC-H cartridges) . The sand which may be used in groundwater monitoring well construction may contain free silica (quartz) . Airborne exposure to silica dust may occur during handling of these sands. Half-face respirators with HEPA cartridges should be worn for those sand handling operations where there is a reasonable possibility for exposure to sustained airborne dust from the pouring and mixing of dry sand. CHEVMERC.H50 18