Additional Monitoring Techniques
In addition to the monitoring methods described in Chapter 7, a host of other techniques exist for monitoring different characteristics of the cabin air or of the occupants themselves. These additional techniques provide more specific information on exposures to cabin air contaminants or on the performance of the environmental control system (ECS) equipment. Some of these techniques have been proposed or used in research investigations of aircraft air quality. These techniques might be considered for subsequent adoption as applications and resources warrant. Several examples of additional monitoring approaches follow.
Methods exist that are specific for phosphorus-containing compounds such as phosphate ester pesticides and organophosphate esters that are used as additives in certain fluids. Instrumentation is sensitive but expensive. Current instrumentation is not well suited to aircraft environments.
Sulfur is present as an impurity in jet fuel. It may impair the function of an ozone (O3) scrubber if aspirated into bleed air flow. Real time sulfur dioxide
(SO2) monitors are available. However, data from such measurements would be of limited utility and do not justify the expense or the use of limited space.
Photoioniziation detectors respond to a large number of organic compounds but are non-specific and have limited sensitivity. In general they are not sensitive enough to detect many of the hydrocarbons at levels that are of potential concern in this setting. Furthermore, these instruments respond with different sensitivity to different compounds, and they are plagued by many of the problems that affect volatile organic compounds (VOC) measurements including drift, surface losses, and marked interference from water vapor. Primarily because of their non-specific response, they are probably of limited additional value unless a measure of total hydrocarbon concentration would be useful.
Catalytic hydrocarbon detectors measure carbon dioxide (CO2) before and after air is passed over a hydrocarbon oxidation catalyst, which provides a measure of oxidizable carbon species. Such systems must be designed to scrub the majority of CO2 from the airstream prior to before/after measurement or signal will be swamped by atmospheric CO2; the same is true of methane and propane. Even taking such precautions, signals will probably still be dominated by ethanol from both breath and beverage service and by acetone from breath. Against this large and varying background, it would be quite difficult to discern changes in CO2 resulting from a condition such as leaked fluid in the bypass air. Consequently, this approach does not appear to be feasible.
In addition to the light scattering instrument discussed in Chapter 7, there are direct-reading particle methods based on the behavior of electrically charged particles (Hinds 1999). Instruments using electrical charge include commercial smoke detectors as well as more technically sophisticated electrical aerosol analyzers. Smoke detectors employ an ionizing radiation source to generate electric charge on particles, and the resulting change in electric current is used to sense the presence of particles in air. These devices respond within seconds to relatively high concentrations of fine particles (e.g., combustion aerosols), but may not be suitable for continuous monitoring of lower levels
aboard aircraft. Electrical aerosol analyzers have the ability to evaluate particle concentration as a function of particle size, and would thus provide useful information about particle size distribution not obtained from the optical devices described earlier. Such instruments are not now available in compact, portable form, and their cost is also likely to prohibit their use in routine monitoring.
Some reports of aircraft crew health problems associated with air quality have suggested that exposure to phosphate ester compounds, such as tricresyl phosphate, or their pyrolysis products may have elicited neurologic symptoms. Exposure data relevant to this question are completely lacking because air monitoring equipment has not been in place on the affected aircraft. One possible solution to this problem may lie in the area of biological monitoring for exposure markers. For example, it has been demonstrated recently that alkyl phosphate compounds are present in the urine of workers exposed to organophosphate pesticides (WHO 1996; Lauwerys and Hoet 1993). These pesticides are chemically similar to the phosphate esters commonly used as additives in hydraulic fluids and lubricating fluids employed in aircraft engines and auxiliary power units, and identified as possible causes of neurological problems in cabin crew members (Centers 1992; Craig and Barth 1999; Crane et al. 1983; Daughtrey et al. 1996; Earl and Thompson 1952; Mackerer et al. 1999; Rubey et al. 1996; Wright 1996; Wyman et al. 1993).
The metabolism of these additives in humans produces alkyl- and arylsubstituted phosphates in close analogy to the metabolic fates of the organophosphate pesticides. Therefore, biological indicators of exposure to the phosphate esters may be available. The metabolites are expected to appear in urine within 24–48 h of exposure, but there is evidence that the metabolites continue to be detectable in urine for as long as 14 days after a single exposure (WHO 1996). The analytical method for these metabolites in urine is very sensitive, with lower limits of detection of 0.05 μmole/L of urine or lower having been reported (Nutley and Cocker 1993). Further, studies in agricultural workers have shown that metabolites can be detected in urine samples from workers who display no symptoms of acetyl cholinesterase inhibition (Nutley and Cocker 1993). Thus, the biological indicator of exposure is useful in identifying workers who have been exposed at levels below those associated with acute clinical effects.
Therefore, implementing a biological monitoring program might be possible based on collection of a urine sample either at the end of a flight, or prior to the start of the next flight, after a possible exposure to phosphate esters or their by-products. It may not be necessary to impose this testing as a routine procedure, but biological monitoring could be used whenever a suspected exposure has occurred and complaints are reported. Although this procedure would provide useful objective information regarding the recent exposure history for each individual providing a sample, biological sampling also has certain negative attributes. These negatives include necessary invasion of privacy, need to obtain informed consent, and the additional effort required to keep confidential the data resulting from analyses (Schulte and Sweeney 1995). Very recent work has suggested that in some instances saliva may be substituted for voided urine as an appropriate sampling medium for biological monitoring (Lu et al. 1998). If this were shown to be feasible for phosphate esters, subjects are likely, to prefer saliva sampling to urine sampling.
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Daughtrey, W., R.Biles, B.Jortner, and M.Ehrich. 1996. Subchronic delayed neurotoxicity evaluation of jet engine lubricants containing phosphorus additives. Fundam. Appl. Toxicol. 32(2):244–249.
Earl, C., and R.Thompson. 1952. The inhibitory action of tri-ortho-cresyl phosphate on cholinesterases. Br. J. Pharmacol. 7:261–269.
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Lu, C., L.C.Anderson, M.S.Morgan, and R.A.Fenske. 1998. Salivary concentrations of atrazine reflect free atrazine plasma levels in rats. J. Toxicol. Environ. Health A 53(4):283–292.
Mackerer, C.R., M.L.Barth, A.J.Krueger, B.Chawla, and T.A.Roy. 1999. Comparison of neurotoxic effects and potential risks from oral administration or ingestion of tricresyl phosphate and jet engine oil containing tricresyl phosphate. J. Toxicol. Environ. Health A. 57(5):293–328.
Nutley, B., and J.Cocker. 1993. Biological monitoring of workers occupationally exposed to organophosphorus pesticides. Pestic. Sci. 38(4):315–322.
Rubey, W., R.C.Striebich, J.Bush, P.W.Centers, and R.L.Wright. 1996. Neurotoxin formation from pilot-scale incineration of synthetic ester turbine lubricants with a triaryl phosphate additive. Arch. Toxicol. 70(8):508–509.
Schulte, P.A., and M.H.Sweeney. 1995. Ethical considerations, confidentiality issues, rights of human subjects, and uses of monitoring data in research and regulation. Environ. Health Perspect. 103(suppl.3):69–74.
WHO (World Health Organization). 1996. Selected pesticides. Organophosphorus pesticides. Pp. 237–251 in Biological Monitoring of Chemical Exposure in the Workplace: Guidelines, Vol.1. WHO/HPR/OCH 96.1 Geneva: WHO.
Wright, R. 1996. Formation of the neurotoxin TMPP from TMPE-phosphate formation. Tribology Transactions 39:827–834.
Wyman, J., E.Pitzer, F.Williams, J.Rivera, A.Durkin, J.Gehringer, P.Serve, D. von Minden, and D.Macys. 1993. Evaluation of shipboard formation of a neurotoxicant (trimethyolpropane phosphate) from thermal decomposition of synthetic aircraft engine lubricant. Am. Ind. Hyg. Assoc. J. 54(10):584–592.