Exposure to methylene chloride II. Psychological functions

Exposure to methylene chloride: II. Psychological functions. health 1 (1975) 95-103. The effect of the solvent methylene chloride on psychological functions such as reac tion time, short-term memory, and numerical ability was studied in 14 healthy male subjects. Each subject was tested repeatedly during exposure to 870, 1,740, 2,600, and 3,470 mg/m 3 of methylene chloride in inspiratory air and during control conditions. Samples of subject alveolar air were taken every third minute during the experiments. No statistically significant impairment in the performance of examined functions could be observed during exposure to methylene chloride as compared to control conditions. In exposure to the highest concentration a greater irregularity of response wa,s obtained for subject reaction time than under control conditions.

tive symptoms similar to those found by these authors have been reported by many authors in conjunction with protracted exposure to methylene chloride (3,10,21). We were able to find only two references dealing with the possible effect of methylene chioride on man's psychophysiological function. DiVincenro et al. (4) found that the ability of subjects to perform simultaneous arithmetic operations and a coordination test was not affected by 4 h of exposure to 350 mg/m 3 and 700 mg/ m S of methylene chloride. Schlipk6ter et al. (16) found that a 4-h exposure to 2,800 mg/m 3 of methylene chloride produced ,a rise ,in the threshold for flicke,r fusion ,and impaired performance in a vigi,lanoe test.
The metabolism of methylene chloride results in an increase in carboxyhemoglobin (COHb) (1). Stewart et al. (18) maintained that exposure to methylene chloride in concentrations close to the threshold limit value may elevate the COHb concentration to more than 5 0/0. The detrimental effect of carbon monoxide on the human nervous system is a ,known fact. However, the question of the COHb levels at which the central nervous system is first ad:fected has been the subject of contention (2,13,14,15,19,20).
The objective of the present investigation was to ,study the extent to which man's cognitive and psychomotor reactions are affected by exposure to methylene chloride. The study was performed, where applicable, according to the same patterns as in previous studies (5,6,7,8).

SUBJECTS
Fourteen healthy men from 20 to 30 years of age were used as subjects. They were all students or employees at the Department of Occupational Medicine.

Experimental design
The subj,ects were divided at random into two equally large groups. Those in one group were 9tudied individually first under experimental conditions with exposure to methylene chloride and then, 7 days later, under control conditions with exposure to ordinary atmospheric air. The subjects in the other group were studied under identical conditions, but in the reverse order. The two different conditions for individual subjects were sta'rted and concluded at the same times of day.
During experimental conditions subjects were exposed for four continuous 30-min periods to 870, 1,740, 2,600, and 3,470 mg/ m 3 of methyl,ene chloride in inspiratory air. The increase in concentration after each 30 min period took place without any break in exposure. The mixture was supplied via a breathing v,alve with very slight resistance. The taste and smell of the gas was disguised by introducing a cannister containing menthol crystals into the tube leading to the mouthpiece. The concentration of methylene chloride in alveoLar air was measured every third minute with gas chromatography.
The study was performed with the subject sitting in a comfortable chair at a desk Four performance tests were carried out in the final 20 min of each exposure period and always in the same order. At given points in each performance test, subject heart rate was recorded with telemetric equipment (Medenik Hon-eywell). The air did not contain any methylene chloride during control conditions. In other respects, all operations and measurements were made with the same equipment, in the same manner, and in the same chronological order as in experimental conditions. Thus alveolar air samples were also taken every third minute.
Information on each subjeot's perception of conditions was collected in the following manner. Immediately after the termination of exposure subjects were asked to assess their condition with respect to the following six variables: calm/hurried, active/passive, relaxed/tense, welldispos,ed/HI..,disposed, unaffected/affected, and alert/tir,ed. Assessments were facilitated with the aid of a 7-point evaluation scale on which extreme values were defined by -opposites. After concluded exposure on the second day, all subjeots were asked if they had been abl,e to notice any difference between the two experiments in respect to the inspir,atory air. The objective of this interview was to determine the extent to which the menthol disguise had been effective.

Performance tests
The following tests were performed in the following sequence during each period of exposure to both methylene chloride and pure air: Numerical ability: RT Addition (RT = reaction time). The test was performed with the aid of a stimulus/response panel with which the lighting of stimulus lamps and subJect re,sponses were electronically recorded. Subjects were ins,tructed to respond to the lamps by touching switches connected to the stimuli. The switches were numbered from 0 to 9 and located in the immediate vicinity of the stimuli, which were also numbered. The test was conducted by -administering a series of three stimuli lasting 0.4 s at 0.1-8 intervals on a panel. Subjects were instructed to add the numbers represented by the stimuli and to report the answer with the aid of the numbered switches. The sequence for the first 4 and the last 16 series varied at random between the different exposure periods. Performance was measured as the mean value of reaction times for the last 16 series.
Simple reaction time: Test 1. Equipment watS the same as in the preceding test. However, this test only utilized one of the stimulus/response units. Subjects were instructed to respond to stimuli by pressing the switch as quickly as possible. The test consisted of a total of 96 stimuli distributed among 6 comparable series, each comprising 16 consecutive stimuli. Each series took about 1 min to perform, md the time elapsing betw-een stimuli varied from 1.7-5.2 s. Performance was measured as the mean value of reaction times for all stimuli. The mean reaction time value for every I-min time-block was also calculated for an analysis of the change in performance over time according to a method developed by Lisper and Kjellberg (12).
Short-term memory. The equipment was the same as in the preceding tests. The test was conducted so that a series of stimuli, which lasted 1 s, was administered at I-s intervals on the panel. Subjects were instructed to touch the switches in the same sequence as the presentation as soon as a signaJ. indicating the end of a series was given. Ten different series, in which the number of stimuliincre.ased frOiII1 4 to 10, were administered to each subject. Response data were analyzed according to two criteria: 1. :the number of cQlNectly reproduced stimuli as a percentage of the total number of stimuli and 2. the number of oorrectly reproduced series as a percentage of the number of series.
Simple reaction time: Test 2. A manually conirolled stimulus/response panel was used. Stimulation was visual, and responses were supplied by touching a switch. Reaction time was recorded with the aid of an electronic counter. Stimuli wer,e ,aJdm1nistered at 10-s intervals, and an acoustic wa,rning signal was supplied 3 s before the administration of each stimulus. A total of 30 stimuli were supplied in each trial. Performance was measured as the mean vaJ.ue of reaction times for the last 20 stimuli administered.
Thus two tests of simple reaction time were used in the present study. One of the tests, designated as Simple Reaction Time Test 1, was especially designed for the study of changes in performance during the 6~min duration of the test. Changes in performance over time have proved to be a useful and sensitive indicator of fiiltigue or laak of sleep (12). The second test, termed Simple Reaction Time Test 2, proved to be the m-ost sensitive of aU tests used in our previous studies of solvents and w,as therefore aJ.so included in the present study.
The other two tests, RT Additions and Short-Term Memory, were used in the study of exposure to white spirit (5). Howev,er, the tests were modified somewhat pri,or ,to the present study so as to tailor them 10 the equipment used.

Concentration in alveolar air during exposure
The concentration of methylene chloride in alveolar samples taken during experimental conditions is shown in fig. 1. The dots in the figuI'e are the mean values of the 14 subjects. Standard deviations varied from 14.8 to 19.9 Ofo of the respective mean values. The figure shows that the increase in alveolar air concentration was greatest at the beginning of each exposure period when no performance measurements were made. The concentration of methylene ch:Ioride in alveolar air was fOUlIlld ,to cOJr:neiLaJte dosely with ,the COl!lcentration in arterial blood (1). Thus:it was possible to obtain a goodapproximation of the concentration of methylene chloride in arterial blood on the basis of alveolar air measurements during the four periods of exposure.
The relation between the conceIlitration of methylene chloride in alveolar air and the concentration in arterial blood for one of the subjects in the physiological experiments is described in fig. 2. The maximal COHb conoentraotion attainable was estimated at 5 Ofoon the basis of the measurements of COHb performed during the physiological experiments (1).

Subjective perception of conditions
The interview results following the experiments were unequivocal. None of the subjects had been able to distinguish between exposure and nonexposure to methylene chloride. Thus disguising methylene chloride in the inspiratory air with menthol had been completely successful.   percepUon of their own condition after the experimental and control conditions, respectively. Subjects' assessment of their condition after exposure to methylene chloride differed systematically from the assessment made following control conditions. A statistical test of the differences between the two conditions was performed using the Wilcoxon matched-pairs signedrank test (17). There were no statistically signif,icant differences (p > 0.05) for any individual variable. However, a high level of significance (z = 2.83; p < 0.005) was obtained for the difference between conditions for the sum of the six variables.
The results suggest that exposure tQ methylene chloride exerted a subj,ectively favorable change in the subjects' experience of condition.

Heart rate during exposure
As shown in f.ig. 4 heart rate, with few exceptions, was lower during exposure to methy1ene ch10ride than under control conditions. However, the differences between conditions were slight and not statistically significant when tested using an ana:lysis of variance.
heart rate

100
In other respects the subjects' heart rates varied in the same manner as in our preṽ ious studies on solvents (5,6,7,8). Thus a systematic reduction in heart rate was found in repeated testing when the same per£ormance test was used along with a variation in heart rate as a function of the nature of the performance tests. This reduction is the most readily apparent in fig. 4, in which heart rate on the different measurement ocoasions displayed the same mutual relationship between the different performance tests.
Performance changes during exposure Table 1 and figs. 5, 6, 7, and 8 describe results for the ,g,roups' perfonnances during experimental and control conditions, respectiv,ely. Figs. 5, 6, 7, and 8 show that in all four tests the change in performance values during exposur,e to the different methylene chloride concentrations did not differ systematica'1ly from the changes noted under control conditions. A statistical test of the significance of mean value differences between experimental and control condi'tions calculated for intrapair mean differ:ences resulted in low, nonsignificant t-values (p > 0.10).

Air
Methylene chloride o RT -ADDITION  a Criterion 1: the number of correctly reproduced stimuli as a percentage of the total number of stimuli; criterion 2: the number of correctly reproduced series as a percentage of the total number of series.
As expected from t.he findings of Lisper and KjoeLlberg (12), there WaJS, under both conditions, a natural decrease in the subjects' reaction time over the I-min timeblocks ( fig. 6). HoweV'er, no differences between oonditions could be found in the rate of decrement. The decrement of the subjeots' r,ea.ct}oo time over the I-min t£me-blocks was most pronounced in exposure periods 1 and 2 and less pronounced in the following two exposure periods. This latter result was probably due to some sort of training effect, which counteracted the decrement in performance over time.
The standard devi,ation for the 96 and 20 S'timuli, respectively, in the two simple reaction time tests was calculated for each subject. '.Dhe obj,ective of these Cialcula'tions 100 was to obtain individual measures of the regularity ,of response on the two tests. Since the standard deviations can vary with the level of performance, they were also calculated as the percentage of each respective mean value.
Both variation measures tended to increase with increasing exposure to methylene chloride as compared to control conditions. However, a stattstical teslt of significance disclosed that the differences between conditions were only significant for Simple Reaction Time Test 1 in ex-pasUire per10d 4 (t = 2.44 and 2.55, l'espectiv,ely; p < 0.05). Thus exposure to the highest concentralJi.on of methylene chloride affected subject reactions in at least one of the reaction time tests. However, sec 1.8 RT -ADDITION this effect was manifested in greater unevenness in subject performance without any change in average reaction time.

METHYLENE CHLORIDE
The objective of the present investigation was to study psychological functions in man during exposure to the solvent methylene chloride. A similar ·experimental design was em,ployed as in our previous studies of the solvents toluene (6), methylohloroform (7), styrene (8), and white spirit (5). These studies examined performance changes and subjective reactions during exposure to .concentrations close to the threshold limit value for each substance. In the present investigation subjects were expOis,ed for a total of 2 h to concentI1ations of methylene chloride corresponding 'to 50, 100, 150 and 200 0/0 of the threshold limit value (500 ppm, i.e., 1,740 mg/m 3 ). Subj,ect performance was studied with respect to reaction time, short-term memory, ,and numerical ,ability. Reaction time was studied using two different tests. One of ,the tests was identical to the one which proved to be especially sensitiv,e to exposure effects in previous studies of solvents. The second test was especia1ly adapted to the study of performance changes over time.
As in our pr,evious studies on solvents the measures adopted to disguise exposure pr.oved to be effective. No subjective symptoms were reported by subjects during interviews aflter the experiment. According to the questionnaire, however, subject assessment ·of their condition after exposure differed systematically from assessments made after control conditions. The results suggest thalt exposure to the solvent had a certain perceptualily positive effect on the subjects' experiences.
No statistically significant impaiI1ffient in the performance of studied functions could be obSJerved .during the 2 h of exposure to methylene chloride as compared to c,ont.rol cor.dimons. Howev,er, exposure to the highest concentrati'on, Le., 1,000 ppm or 3,480 mg/m 3 , produced certain indications pointing to a commencing detrimental effect of ;tlhe solvent. Thus performance on one of the tests of r·eaction time, despite unchang,ed average performance, was found to be significantly mo~e irregular during exposure than under control condim'ons. A simi1ar change in psychomotor behavior was observed by Gamberale ·and Svensson (9) in anemhetic nurses exposed to low concentrations of anesthetic gases.
An analysis of the results as a wh.ole suggests that brief exposure to the solvent methYJlene chloride at concentrations at or immediately above the present Swedish threshold limit va,lue has no detrimental ·effect on man's psychomotor and cognitive functions.
In the present .study, during exposure subjects attained ,a COHb level approalChing 5 %. Thus the results also suggest that a COHb lev,el of this magnitude does not have a detrimen1Jal effect on the studied functions.