Working at high altitudes induces a variety of biological responses, as described elsewhere in this chapter. The hyperventilatory response to altitude should cause a marked increase in the total dose of hazardous substances which may be inhaled by persons occupationally exposed, as compared to people working under similar conditions at sea level. This implies that 8-hour exposure limits used as the basis of exposure standards should be reduced. In Chile, for example, the observation that silicosis progresses faster in mines at high altitudes, led to the reduction of the permitted exposure level proportional to the barometric pressure at the workplace, when expressed in terms of mg/m3. While this may be overcorrecting at intermediate altitudes, the error will be in the favour the exposed worker. The threshold limit values (TLVs), expressed in terms of parts per million (ppm), require no adjustment, however, because both the proportion of millimoles of contaminant per mole of oxygen in air and the number of moles of oxygen required by a worker remain approximately constant at different altitudes, even though the air volume containing one mole of oxygen will vary.
In order to assure that this is true, however, the method of measurement used to determine the concentration in ppm must be truly volumetric, as is the case with Orsat’s apparatus or the Bacharach Fyrite instruments. Colourimetric tubes that are calibrated to read in ppm are not true volumetric measurements because the markings on the tube are actually caused by a chemical reaction between the air contaminant and some reagent. In all chemical reactions, substances combine in proportion to the number of moles present, not in proportion to volumes. The hand-operated air pump draws a constant volume of air through the tube at any altitude. This volume at a higher altitude will contain a smaller mass of contaminant, giving a reading lower than the actual volumetric concentration in ppm (Leichnitz 1977). Readings should be corrected by multiplying the reading by the barometric pressure at sea level and dividing the result by the barometric pressure at the sampling site, using the same units (such as torr or mbar) for both pressures.
Diffusional samplers: The laws of gas diffusion indicate that the collection efficiency of diffusional samplers is independent of barometric pressure changes. Experimental work by Lindenboom and Palmes (1983) shows that other, as yet undetermined factors influence the collection of NO2 at reduced pressures. The error is approximately 3.3% at 3,300 m and 8.5% at 5,400 m equivalent altitude. More research is needed on the causes of this variation and the effect of altitude on other gases and vapours.
No information is available on the effect of altitude on portable gas detectors calibrated in ppm, which are equipped with electrochemical diffusion sensors, but it could reasonably be expected that the same correction mentioned under colourimetric tubes would apply. Obviously the best procedure would be to calibrate them at altitude with a test gas of known concentration.
The principles of operation and measurement of electronic instruments should be examined carefully to determine whether they need recalibration when employed at high altitudes.
Sampling pumps: These pumps usually are volumetric—that is, they displace a fixed volume per revolution—but they usually are the last component of the sampling train, and the actual volume of air aspirated is affected by the resistance to flow opposed by the filters, hose, flow meters and orifices that are part of the sampling train. Rotameters will indicate a lower flow rate than that actually flowing through the sampling train.
The best solution of the problem of sampling at high altitudes is to calibrate the sampling system at the sampling site, obviating the problem of corrections. A briefcase sized bubble film calibration laboratory is available from sampling pump manufacturers. This is easily carried to location and permits rapid calibration under actual working conditions. It even includes a printer which provides a permanent record of calibrations made.
TLVs and Work Schedules
TLVs have been specified for a normal 8-hour workday and a 40-hour workweek. The present tendency in work at high altitudes is to work longer hours for a number of days and then commute to the nearest town for an extended rest period, keeping the average time at work within the legal limit, which in Chile is 48 hours per week.
Departures from the normal 8-hour working schedules make it necessary to examine the possible accumulation in the body of toxic substances due to the increase in exposure and reduction of detoxification times.
Chilean occupational health regulations have recently adopted the “Brief and Scala model’’ described by Paustenbach (1985) for reducing TLVs in the case of extended working hours. At altitude, the correction for barometric pressure should also be used. This usually results in very substantial reductions of permissible exposure limits.
In the case of cumulative hazards not subject to detoxifying mechanisms, such as silica, correction for extended working hours should be directly proportional to the actual hours worked in excess of the usual 2,000 hours per year.
Noise: The sound pressure level produced by noise of a given amplitude is in direct relation to air density, as is the amount of energy transmitted. This means that the reading obtained by a sound level meter and the effect on the inner ear are reduced in the same way, so no corrections would be required.
Accidents: Hypoxia has a pronounced influence on the central nervous system, reducing response time and disrupting vision. An increase in the incidence of accidents should be expected. Above 3,000 m, the performance of persons engaged in critical tasks will benefit from supplementary oxygen.
Precautionary Note: Air Sampling
Kenneth I. Berger and William N. Rom
The monitoring and maintenance of the occupational safety of workers requires special consideration for high altitude environments. High-altitude conditions can be expected to influence the accuracy of sampling and measuring instruments that have been calibrated for use at sea level. For example, active sampling devices rely on pumps to pull a volume of air onto a collection medium. Accurate measurement of the pump flow rate is essential in order to determine the exact volume of air drawn through the sampler and, therefore, the concentration of the contaminant. Flow calibrations are often performed at sea level. However, changes in air density with increasing altitude may alter the calibration, thereby invalidating subsequent measurements made in high altitude environments. Other factors that may influence the accuracy of sampling and measurement instruments at high altitude include changing temperature and relative humidity. An additional factor that should be considered when evaluating worker exposure to inhaled substances is the increased respiratory ventilation that occurs with acclimatization. Since ventilation is markedly increased after ascent to high altitude, workers may be exposed to excessive total doses of inhaled occupational contaminants, even though measured concentrations of the contaminant are below the threshold limit value.