Objectives-To investigate the occurrence of sick building syndrome in a tropical city, and its relation to indoor air quality and other factors. Methods-2856 oYce workers in 56 randomly selected public and private sector buildings were surveyed. The study consisted of a self administered questionnaire assessing symptoms and perception of the physical and psychosocial environment, inspection of the building plans and premises, and measurement of temperature, relative humidity, respirable particles, chemicals, bioaerosols, and other variables. Results-Symptoms typical of the sick building syndrome were reported in 19.6% of the respondents. Multivariate modelling substantiated contributions associated with low thermal comfort, high work related stress, too much noise, a history of allergy or other medical conditions, poor lighting, young employees, and female sex. Measurements of indoor air quality or ventilation were not found to be reliable predictors of the symptoms. Conclusion-The survey confirmed the presence of sick building syndrome and its risk factors in the tropics. A biopsychosocial approach to the problem involving symptomatic treatment, environmental control, good ergonomic design, and stress management is recommended. (Occup Environ Med 1998;55:188-193)

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Neurobehavioural tests were performed by 98 dentists (mean age 32, range 24-49) exposed to elemental mercury vapour and 54 controls (mean age 34, range 23-50) with no history of occupational exposure to mercury. The dentists were exposed to an average personal air concentration time weighted average (TWA) of 0-014 (range 0-0007-0-042) mglm3 for a mean period of 5-5 (range 0 7-24) years and had a mean blood mercury concentration of 9-8 (range 0-6-57) pgIl. In neurobehavioural tests measuring motor speed (finger tapping), visual scanning (trail making), visuomotor coordination and concentration (digit symbol), verbal memory (digit span, logical memory delayed recall), visual memory (visual reproduction, immediate and delayed recall), and visuomotor coordination speed (bender-gestalt time), the performance of the dentists was significantly worse than that of the controls. The dentists scored 3-9 to 38-9% (mean 13-9%) worse in these tests. In trail making, digit span, logical memory delayed recall, visual reproduction delayed recall, and bender-gestalt time test scores were more than 10% poorer. In each of the tests in which significant differences were found and in the block design time, the performance decreased as the exposed dose (product of the TWA of air mercury concentrations and the years of exposure) increased. These results raise the question as to whether the current threshold limit value of 0 050 mg/m3 (TWA) The organs most frequently affected by metallic mercury in chronically exposed subjects are the nervous system, kidney, and mucosal surfaces of the mouth.'2 The central nervous system is probably the most sensitive organ."5 Many studies on the neurotoxic effects of mercury have been reported."3 The earlier ones were usually on subjects exposed to high concentrations (more than 0 05 mg/m3) of mercury.57 Some of the more recent studies generally lack adequate measurements on exposure to mercury.4 12Neurobehavioural performance tests are sensitive methods used to detect subclinical effects.9'0 The purpose of this study was to measure early changes in performance of motor speed, manual dexterity, visual scanning, and visual and verbal memory among dentists exposed to mercury vapour at concentrations of less than 0-05 mg/m' in their work environment. Materials and methods STUDY POPULATIONA cross sectional study of 98 dentists (representing 32% of active dentists in Singapore), 38 women and 60 men exposed to a time weighted average (TWA) concentration of air mercury vapour below 0-05 mg/ m3was undertaken. The dentists typically worked 10 hours a day for a six day work week in air conditioned rooms. A control group of 54 persons (27 women and 27 men) with no history of occupational exposure to mercury were selected from staff at the National University of Singapore. Questionnaires were used to select subjects (dentists and controls) that did not have a history of neuropathy from diabetes, renal diseases and excessive alcohol use, disease of the central or peripheral nervous system or psychiatric d...

Neurobehavioural tests were undertaken by 30 female workers exposed to toluene and matched controls with low occupational exposure to toluene. The environmental air levels (TWA) of toluene was 88 ppm for the exposed workers and 13 ppm for the controls. The toluene in blood concentrations for the exposed workers was 1-25 mgIl and for the controls 0-16 mg/l. Statistically significant differences between workers exposed to toluene and controls in neurobehavioural tests measuring manual dexterity (grooved peg board), visual scanning (trail making, visual reproduction, Benton visual retention, and digit symbol), and verbal memory (digit span) were observed. Further, the performance at each of these tests was related to time weighted average exposure concentrations of air toluene. The workers exposed to toluene had no clinical symptoms or signs. The question arises as-to whether these impairments in neurobehavioural tests are reversible or whether they could be a forerunner of more severe damage.

Quantitative theories based on first principles are derived to explain the roles of heat and mass transfer in the attainment of steady state or ultimate separation in closed direct-thermal-mode parametric pumps. The theoretical results are in agreement with applicable experimental data, including the experimental anomaly called reversed separation. Under conditions when the axial dispersion is much greater than molecular diffusion, the theory predicts and experiments confirm that l/ln(a,) varies in a linear way with fluid displacement at constant frequency. An additional asymptotic result suggests that when Sh >> PeM, there is a simple linear connection between l/ln(a,) and PeM which is supported by experimental data. , 1975). Moreover, the question of phase difference has never been given a firm theoretical footing. In the present study we undertake to answer these questions.In the direct-thermal-mode of operation, a solid ad-:orbent with a temperature-dependent affinity for the solute to be senarated is used as a retardation media, while periodic fluid motion pumps the lean solvent phase (co'd cycle) and rich solute phase (hot cycle) to their respec5ve reservoirs. During the first part of a cycle, the steps involved include first the transient cooling of the packed bed; this causes solute migration from the bulk phase to the solid adcorbent. In this journey, the solute is retarded by a film resistance at the surface of the solid and disper.zed by the action of the anplied fluid motion. Some rolute moves into the solid by pore diffusion, adsorbing on the pore walls along the way.In the second part of the cycle (hot cycle), the solute de-orbs and wends its way back to the bulk phase in exact reverse of the preceding steps. The rate of solute movement depends on the local bed temperature, which determines the equilibrium driving force for mass transfer. This fundamental picture has been recently used (Rice, 1975) to evolve a quantitative theory to predict the magnitude of concentration difference at ultimate separa!ion (1 --i; n) between the lean and rich reservoirs in a parametric pump. Notwithstanding the reasonable comparison with experiment using only parameters calculated from first principles, there are serious questions regarding the generality of this theory, especially the assumption that the axial concentration profile is linear. Furthermore, the aforementioned theory precludes the occurrence of an equilibrium pinch type of thermodynamic l'mitation, since the form taken by the linearized adsorption isotherm is zuch that at different temperatures, isotherms are required to be parallel.The analytical model presented in the present study differs from the former approach in several respects. While building on the apparent successes of the earlier model (Rice, 1975), we derived a more realistic axial concentration profile to show an exponential dependence. Furthzrmore, equilibrium isotherms were not assumed to b,-parallel. Finally, as a strong test for the new theory, the curious reversed-se:>aration pheno...