Exposure to methylene chloride. I Its concentration in alveolar air and blood during rest and exercise and its metabolism.

A. Exposure to methylene chloride: 1. Its concentration in alveolar air and blood during rest and exercise and its metabo lism. Scand. j. work environ. & health 1 (1975) 78-94. Fourteen subjects were exposed to about 870 and 1,740 mg/m 3 of methylene chloride in the air during rest and phys ical exercise on a bicycle ergometer. The duration of each exposure period was 30 min. Each subject was exposed during four periods. The concentration of methy lene chloride in the alveolar air increased in the beginning but had a tendency to level off at the end of each period. There was a high correlation between the alveo lar and arterial concentration of methyLene chloride. The uptake of methylene chloride was about 55 Ofo of the supplied amount at rest, about 40 Ofo at a work load of 50 W, and about 30 and 35 Ofo at 100 and 150 W, respectively. The concentration of carboxyhemoglobin (COHb) increased both during and after exposure. With expo sure to 1,740 mg/m 3 a concentration of COHb in the blood of about 0.85 g/100 ml was reached. This value corresponds to about 5.5 % COHb.

Methylene chloride (CH 2 CI 2 ) is a solvent in relatively widespread use. About 2,300 tons are good in Sweden annua:lly. It is used a'S a solvent f.or dyes in the dyeing industry, as a dissolver of cellulose esters in the textile industry, for the extraction of fats in the food industry, as a refrigerant in air conditioning units, ,as a degreasing agent in the engineering industry, etc. Thus a great many workers come in contact with the substance in their work.
78 er things, that the concentration of methy;lene chloride in alveolar air rose very rapidly during the first minutes of exposure; the concentration increased more slowly during the subsequent 40-50 min. A further decline in the rate of concentration increase was also noted in the following hour. The concentration in venous blood rose more slowly and tended to stop increasing .after about 2 h of exposure.
These authors had exposed subjects to concentra'tions nea,r the present (December 1974) Swedish hygienic limit value, i.e., 500 ppm, which corresponds to 1,740 mg/ m 3 at 25°C. But von Oettingen (17) used very heavy exposure (up to 40,000 ppm) and found that blood concentration remained stable near a given level for about 1.5-2 h during anesthetic experiments with animals.
A similar pattern for the concentration in alveolar air and blood was found by Astr'and et al. in studies of toluene (1), methylchlorcrlQI1m (4), and ,aliphattc components in white spirit (2). The palttexn was eXlPlained in the study of white spdrit by the relatively poor sdlubtlity of the sdlvent in blood. MethYlleneohlol"ide also appears to dissdLve poorly in ,blood. TheTefore, it ,shou1!d be comparatively innocuous if the toxicity is low.
Stewart & Dodd r (23) efound that a soLution of methylene ,ohLoride rs liIbsorbed through the skin. RQWeVer,Slkin is damaged relatively qULckly in this tY[>e of ex:posure and Tesul1ts in pain. Thus this kind of eXiposUire is ,probably insignificant in orldinary indUistriailcontexts, since pain would automatically serve to warn against fUirther ,eXlP'osure.
,In 'the present investig.ation sUlbjects were eXiposed to me1JhY'lene ch'loride both at rest and during ,exerdse in the same manner as in pr,evious studies made ai 'the National Board of OccUIPational Safety and Health 1(1, 2, 3, 4). 2

SUBJECTS
Fourteen men 19 to 29 years oif ag,e served as sUl~ects. Twelive werestudenlts. The sUlbjects were given 'a careful clinical examination. The function of their respkatory andcil1Cu1atory organs was tested both at re,st and during exercise. The same methods were used in the medical exammationas in previous studies of other solvents (1,2,3,4).
All the sUlbjects were healthy at the time of the examinlaltion, and none of them had ever sUI£fered from any disease having a detrimental effect on respirwtory or ciI1cula1tor,y OI1gans. Values fOil" hemoglobin CODicentral1Jion, hematocrit, and erythrocyte sedimentation rate 'were 'Within normal limits for a111 subjects. No one 'had ail-bUlffiin, blood, or reducible sUibstances in their urine. Resul1ts from the lung function ,tests were also nOI1ffial1.Five subjects were smokers. They were ,aslked not to smolke in the 12 h prior to tlhe expOSUT,e eXlperimenlt. Resu:l.ts from measurements made dJuI1irrg thie :medkal ,examination are listed intlab'le 1.
T,aibile 2 reports the mean values o[ measurements talken from the subjects during exercise on a bicyrcle ergometer without eXiposure. AI'! the values :lirom submaximal and crnaximwl exercise were nOI1mal. Thus the subj,ects of 'this investtgation responded nOI1mally to Phy,sicall exercise and displayed ,a nOII1ma[ IPhiysic.al work ,capacity. There was no significanidif[,erences between mean values in taMes land 2 for subjects in this study and for subjects in studies previously per:£O'rmed (l, 2, 3, 4). Ther,efore, groUIP di£fier,enoes ,in reactions to dif[erenJt solvents <oannot be ascdbed to physiologkal differences between subjec.ts.
Occwsional ectQPic beats were recorded in the resting eleotrocardiogram (ECG) o[ or~e ISUlbject. Andther sUlbject dispilayed sl~ght ST depression with ,segmental changes and fl.iattening lof the T wave during exercLse withoUit any other signs of the heart being aJffected. There were no other signid'icant EGG ohanges.

EXRERLMENTAL DESIGN
Subjects were eXiposed during rest and ex'ercise to methytlene ahlor.ide concentrations in inspirartory air which were dose to, or about half of, the threshold limit value (TLV). The Swedish TLV for methylene chlorid~is 500 ppm, corresponding to about 1,740 mg/m 3 at 25°C. As of January 1, 1975, however, the Sw~dish TLV will be 100 ppm (350 mg/m 3 ). All air concentrations are henceforth reported in mg/m 3 values at 25°C unless otherwise specitied. The air mixture was prepared in a manner similar to that described in previous studiJes ( fig. 1).
The concentIiation of methylene chloride in inspiratory air was continuously fOdlowed with a gas indicator (Hydrocarbon analyzer, Model 116, Scott Research Lab. Inc., RlumsteadviIJ.e, Pa., U.S.A.). Any given methylene chloride 'concentration in air warS prepared with accuracy sufficient to produce 1,690 to 1,790 mg/m 3 when 1,740 mg/m 3 was the objectiv,eailld 820 to 920 mg/m 3 when the objectiVle was 870 mg/ m 3 . The experiments were performed along the same general lines as in previous studies of white spirit and styrene (2,3). At the start arterial and venous catheters weI"e introduced into a brachial artery and a medial cubital vein. The subject was then exposed. Each period of exposure lasted 30 min, and each subject was exposed on the same occasion in four con-secutive periods, Le., a total of 2 h each.
The subjects were exposed according to the following three alternatives ( fig. 2). Series I: Five subjects were exposed to both about 870 and about 1,740 mg/m 3 of methylene chloride in inspiratory air during rest (30 + 30 min) and during exercise (30 + 30 min) at an intensity of about 50 W (300 kpm/min). Series II: Four subjects were exposed to .about 870 mg/m 3 of methylene ohlo.ride in inspiratory ai.r during rest (30 min) and during exercise (30 + 30 + 30 min) at an intensilty of about 50 W (300 kpm/min). Series III: Five subjects wer,e eXipos,ed t,o about 1,740 mg/m 3 of methYllene chloride in inspiratory ak during rest (30 min) ,and during exercise (30 + 30 + 30 min) at intensities of about 50 W (300kpm/min), 100 W (600 kpm/min), and 150 W (900 kpm/min). Fig. 3 shows the times at which the measurements were made and the tests ta'ken. Alveolar air samples for methylene chloride assay were taken from the breathing vaJl.ve with the us,e of a glass syringe during exposure and in glass tubes after exposure. AI1terial and venous blood samples (approx. 0.5 g) were taken from the catheters and collected in 25-ml glass bottles for the analyses of methylene chloride and collected in 10-ml glass bottles  Fig. 1. The air mixture was produced as follows: Compressed air was led through a charcoal filter (A) to rotameters (B and C) connected in parallel; both rotameters were fitted with valves to regulate the air flow. Air passed from one rotameter (C) into a wash bottle (D) containing methylene chloride. In the bottle (D) there was a tube (E) containing a capillary tube (F). The methylene chloride was ascending in the tube (E), and the air passed through the capillary tube (F). Air and the methylene chloride mixture were then fed to a closed vessel (G) in which mixing took place. Thereafter, the air mixture was conducted to the base of a cylinder (H) from which inspiratory air was sucked into the breathing valve via a metal tube (I). Excess gas was removed through a chimney (K). The supply of air containing methylene chloride was delivered to the cylinder (H) at a rate of 60 to 100 lImin and was never less than a subject's pulmonary ventilation. The device was placed in a fume cupboard, and excess gas was exhausted.
for COHb assay. Preceding studies on solvents provide the details of these techniques (1,2,3,4). A mean value for the concentration of methylene chloride in alveolar air was calculated for each subject and each period on the basis of the final three determinations. The concentration of methylene chloride in aI1terial and v-enous blood was calcubted as the mean value for each subject on the basis of the final two determinations in each period. These concentrations tended to be at a given level at the end of each period. However, the COHb concentration increased continuously. Thus the highest COHb value (generally the final value) was selected for each subject and period.
The concentrat.ion of methylene chloride in alveolar air and blood and the concentration -of COHb in blood were followed for another 2 h after the conclusion of the four periods of exposure. The exact times fm oSaIIJiPliIJ.@S are shown in fig. 3 ·and in the fig1.lJl"es describing the results. The final value in each 30-min periJOd for eaoh subject was used in calculating the mean values for the respective concentrations 30, 60, 90, and 120 min after concluded exposure.
The ECG was recorded continuously during exposure, and heart rate was determined every other minute during these recordings. The mean V'alue for the three final determinaJtions during each exposure period was used. Blood samples for lactic acid assay w.ere ta!ken a<t the end of each period.
The volume of expiratory air was con-tinuousJy measured in bags (specia.J.ly made of polyester-laminated aluminium foil) throughout the entire exposure, i.e., for 2 h, and the methylene chloride content of expiratory air was determined. The rigures -describ1ng iI',esults show how the expired air was fractionated in 20 to 30 different bags in the thl'ee types of exposure experiments. The volume of inspirart:ory air was estimated to be the same as the volume of expiratory air, and the uptake of methylene chloride in the organism was ca.J.culated as the difference between the total amounts in inspiratory and expiratory air. The alveolar ventilation per minute was calculated for the latter half of each period, i.e., for about 15 min.
The oxygen content of expiraitory air was analyzed, and oxygen uptake was calculated for ,the latter half of each exposure peri·ad at rest and for the final 5-10 min of exercise. The mean value for the number of determinations (bags) was used.
These latter measurements, as well as the determination of blood lactic acid concentration, were made in order to facilitate the assessment of exercise severity for ea'ch subj.ect.

ANALYTICAL METHODS
Respiratory volumes, blood lac,tate con-centra<tion, and heart rate were determined according to methods described in the toluene study (1). The oxygen and carbon dioxide content of expiratory air was determined with an oxygen analyzer (Beckman model E 2) and a carbon dioxide analyzer (Beckman model LB 2), and oxygen uptake was oalculated. od error for these analYlses was indicated in the white spiJrit study (2). A1JvIeo[ar veIlitiJ]iat~on (VA) was calculated 'Oll the basis of pu'lmonary ventilation eVE), Ilespirato.ry rat,e, and dead Ispace (VE-respiratory rate' dead space = VA)' Respiratory rate was recorded by using the ECG stvilp ohart reoorder with the aid o.f a heat recepto,r located in the breathing Vlalve. Dead space in subjects, including ,the valve, was set at 150 crn 3 for all subj eots. iNo oorrections were made for any laIlger dead space occurI'ling during exercise, siThce differences at the ven'tilations measured are oomparatively slight.
The methyJ.ene cModde content was ca!lculated on the basis of the Clhromatogr,am with the aid of standard air samples containing known amounts of methyilene chl1oride.
The methyllene ohlodde oo.ntent o'f blood was determined with a "head space" method (4). The assay was performed by using a gas ohromatogr'CllPh (Model F 30, FID, Perkin-E1:mer Ltd., BeaconsfieiLd, Buck-ing·hamshire, England) equipped with a stainless steel SCOT column (15-m lon,g, 0.51-mm inner diameiter) wiJth Ca,rhowax 400 as ilhe strutionaI'IY phase. 'Dhe carrier gas flow was 4 ml/min, and the column teIIljperature, 80°C. The methyilene clrloride content of the head space was calculated Lvom indiJVlidUJa,1 Mood samples and stamdard air samples containing 'known amounts of methyiJ.,ene ICIhlo,riJde. 'l1he er,vor of the method for a single determination was calculated on the basis of 10 double determinations with blood in which methylene chloride ,contents ranged fr{~m 2.9-3.4 rngl g of blood and amounted ,to ±7.6 0/0 of the mean value.
The COHb content in blood was determined w,ith a method described by Q,vr,um (18). A 0.2-mlcitrate solution, as an anticoagulant, and 1 dr,op of ootyl alcohol, as an antifoaming ag1ent, were added to a 10-mll. ilnijection botHe. The hottle, 'indud-ing a SBrum cap (tubber membrane), was weig1hed before and after the addition of 0.5-1.0 m1l 'Of Wood.
The rUlbber membrane sealing a bottile containing a blood sample was pierced. w.i.th a needle connected via a tube to a water suction unit. The needle was removed after 10-20 s, and 3.0 ml of r,eagent solution (2 pal'its 3.2 potassium ferricyanide + 8 % S3IPon~n in walter + 1 part 0.8 % l'actic acid in water) was trans.:reNed Woitih a syringe Ito the bottle by piercing the membrane. Mter removal of the needle the bottle was shaken for 20 min in a iliaJker. Thereafter the membrane was pier.ced with a needle so as to equalize the pressure. A needle connected to a plastic or latex tuibe [ilIed wd:th water was inserted thTo~h the membrane.
The other end od' l1Jhe 1lube was submel1ged in a water-fmed beaker. With a gas,tight syringe exactly 1 mil of air was fuen removed fram the bottle. 'I1he negative pressure in the bottle was simultaneously equailized when a oorresponding volume of water was suclred inlo the bottle. The syringe tip was withdmwn when the influx ill water ceased.
The sample in the syringe was then injected into a gas chromatograph (Model F 11, with a thel1ffia:1 conductivity detectoT, Perkin4Elmer :Utd., Beaoonsfi~ld, Buckinghamshire, EnJg)1and) equipped with a stainless steel column (2-m long, 2.2-mm inner diameter) Ifiilled with mo!lecular sieve 13 X i(60-80 mesh). Helium (30 mJ1/min) was used as the carrier gas, and the column temperature was 40°C. A standard gas (1 mI) with a CO content of the same magnitude as ·the samples was used in the evaluation 'of (;oncen<trati'ons.
'llhe following formU!la was used in calcU!lating the concentration of the carboxyhemoglobin: % CORb = 6.82· ,a· VI - in wlhich a = the volume of CO per volume of air in standard gas, V I = bottle voiume (ml), V 2 = the volUlffie of rea,gent solution, Va = ·the blood volume, V 4 = the volume of oitratte soliul1;ion + ootyl alcohol, !h = hemoglobin content (g Hib/100 ml of blood), tSaJlll(ple =tJhe peak hdght for CO in tJhe sample's gas ohromatogram, tstandard = the peak height for 00 in the ohromaltogram of the standal1d gas. 'Ilhe er,ror of the method for a single detel1ffiinailllon, cailcu:lated from 10 double detemninations, was 3 % of the mean vaIlue of 4.25 % CORbo 'I'he hemoglobin con1;enJt of blood was determinJed before and af.ter exposure by speotrqphQ'tometri{; means with the cyanmethemoglobin method.

Pulmonary ,ventilation and blood circulation
The SUlbject who d~ayed occasional ectopic beats in his resting ECG during the medioal examination also displayed occasional eotopic beats during eXiposure.  Prior to exposure two othex subjects showed a'bri,aJ1. xhythm ,arising alternately from two different foci. This arrhythmia persisted durJng exposur,e at rest, but disappeaxed during exercise. The subject who displayed ST depression in the medical examination in conjunction with exercise also di,splayed the same type and degr>ee of change during exposure and exercise. In conjunction with exercise and exposure two other subjects developed ST depressions and segmental changes. They displayed no other signs of effect on the heart. Values for alveolar v,entilation, oxygen uptake, and heart ra,te during exposure at rest were of a nOI1mal magnitude (table 3). The values dur,ing 50 W (300 kpm/min) work failed to differ systematiicailly from the corresponding va1ues in the medical examination without exposure (tables 2 and 3). The subjects who were exposed during ex;erdse at 100 and 150 W (600 and 900 kpm/min) displayed a physical work capacity which was above average for the group. This circumstance was reflected in lower than average values for heart rate and blood lacbate concentration (tables 2 and 3). The slow increase in heart ra,te and oxygen uptake noted in the four subjects exposed during 50 W exercise (300 kpm/ min) for 1.5 h was moderate and probably unrel,ated to exposure.
No differences were found in alveolar ventilation, oxygen uptake, and heart rate either at rest or during exercise at an intensity of 50 W (300 kpm/min) between exposure to about 870 mg/m 3 or to about 1,740 mg/1IIl 3 (table 3).
Aft an exerctse intensity of 50 W (300 kpm/min) the suhjects utilized an average of 27 Ofo of theIr maximal aerobic work capacity (max V0 2 ), about 45 Ofo at 100 W (600 kpm/min), and about 64 Ofo at 150 W (900 kpm/min). Lactic acid concentrations in blood at corr-esponding intensities suggested that 50 and 100 W can be regarded as relatively light work, and 150 W, as relatiViely heavy work (tables 2 and 3). spiratory air (series I and II) the ooncentration in alveolar air amounted to about 250 mg/m 3 or about 30 % of the concentration in inspiratory air (table 3). The corresponding arterial blood concentration was about 2.3 mgfikg. The alveolar concentration nearly doubled, rising to 480 mg/m s and corresponding to about 55 % of the concentration in inspiratory air, whereas the arterial concentration somewhat more than doubled, rising to about 5.1 mg/kg duri.ng eXJercise at 50 W. The alveolar ventilation increased about threefold during the corn~sponding work load ( fig. 4 a).
The alveolar and arterial conc.entrations slightly more than doubled in exposure at rest to about 1,740 mg/m 3 of methylene chloride (series I and III) as compared to the values recorded in exposure to half that con<:entration (table 3). During work at 50 W both alveola.r and arterial concentrations increased in about the same way a,s in exposure to the lower methylene chloride concentration. The alveolar concentration amounrt:ed to approximately 55 % of the concentration in inspira1tory air, and the l"esting arterial concentration rose from 5.5 mg/1kg of blood to 10.6 mg/kg ( fig. 4 a).
In work at 100 aoo 150 W alveolar ventilation increased fivefold and sevenfold, respectively, when compared to resting conditions. On the other hand, alveolar concentration only increased slightly, i.e., to 1,090 and 1,220 mg/m 3 , respectively, corresponding to 63 and 70 010, respectively, of the concentration i.n inspiratory air (series III). Equivalent arterial concentrations rose to 13.4 mg/kg and 14.8 mg/kg of blood, res.pectively (table 3, fig. 5 a).
During the mentioned periods the increase in alveolar and arterial concentrations tended to decline towards the end, i.e., after 25-30 min of each period. However, this fla1tening of the curve s.lope became fa.r more striking during the third and fourth periods of senes II, wh~Clh comprised constant exposure and exercise for 90 min at a wOl"k intensity of 50 W (table 3, fig. 6 a).
The relationship between arterial and alveolar concentrations of methylene chloride at the end of each exposure period was linear ( fig. 7).

Venous concentration and arteriovenous methylene chloride difference during exposure
The venous concentration of methylene chloride generally followed the arterial concentration (figs 4 a, 5 a and 6 a). The arteriovenous difference, which to some extent reflects the release of methylene chloride to other organs, was about twice as great in exposure to the higher concentration as in exposure to the lower concentration at rest (table 3). This difference increased further in work at 50 and 100 W but dropped at 150 W to about the same value as at rest (table 3). Release from arte.riail blood per unit of time, expressed in mg/min, was naturally higher at 150 W than at 100 W, since cardiac output was then £ar greater. Cardiac output has been measured in previous studies (2,3,4). It then amounted to 5 l/min at rest, 10 l/min at 100 W, and about 18 llmin at 150 W.
However, it should be pointed out, as in the previous studies on solvents, that the venous blood sampled was peripheral and not central. It is conceivable that peripheral blood does not provide the same information during rest as during exercise in view of the redistribution of blood ciJrculation which takes place in oonjunction with the transition from rest to work.

Metabolism of methylene chloride
Figs. 4a, 5 ia,amJd 6ashow1Jhat ,the COHib concentra'tion increased both during the course of eXiPosure and after the end of exposure. The :finaJ determinations were made 2 h after the end of expOIsure. However, the rate of increase declined towards the end of the 2 h. This decline suggests that peak values were achieved in most cases. [Stewart et al. (24,25) arrived at the same results in 1972.] It wasatlso found that the venous concentrations did not differ systematically from arterial concentrations. For technical reasons more arterial samples we,re taken than venous samples. Therefore, only arterial concentrations are noted in the tables. Table 6 reports the mean value for the highest (fina,l) COHb concentrations in each 30 min ex;posure period. The values are corrected for the resting value,s, which averaged 0.12 g/100 ml. Four of the five subjects who reported being Siffiokiers had eleva't,ed resting v,alues prior to exposure (0.24, 0.14, 0.34 and 0.22 g/100 ml). In exposure ,at rest to about 870 mg/m 3 of methylene chloride (series I) the concentration rose to 0.12 mg/100 ml from 0 and £urther ,to 0.26 g/100 ml in the transition to double the concentration. The COHb content was less than expected in the fourth period during exerdse at an intensity of 50 W with a doubling of the 88 uptake of methylene chloride. The COHb concentra'1Ji'on in series II remained about the same during pedods 1 and 2 as in series I. The concentration increased slowly in continued work at 50 W for an additional hour. The value was low for period 1, in which Ithe expOtsure was the higher concentration, in series III. During wo:r!k at 50 W ,the concentration increased relatively little. Only a small relative increase occurred also in the last two periods wHh exeroise at 100 and 150 W. After the end of exposure the COHb level rose more in sertes III than in the other two series (table 5). All three series are illustrated in fig 10. The uptake of methylene chloride was rar greater during the exercise periods than during the rest periods (table 4). The arterial blood concentration was also higher during exercise than at r,est. If CO is assumed to be derived from the metabolism of methylene chloride and the degree of metabolism is constant, the COHb level should increase, after a slight delay, in step with f1;he uptake of methylene chloride. Values lesls than expected during the fourth period of series I and III may, however, have been due to the flushing oulj; of  CO from the a1lveoli in conjunction with the increased pulmonary ventilation which occurred during exercise. Values greater than expected, especially 30 min after the end of exposure in series I and III, may analogously be due to the reduced ventilation at rest and the attendant low level of "a·iring." In conjunotion with exposure to the TLV, extra CO was produced in quantities corresponding 'to a blood COHb level of 0.7 g/ml. In aJddition ther·e was a resting value of mor·e than 0.1 g/100 ml, Le., an aggregate of 0.85 g/100 ml. This value corresponds to about 5.5 0/0 COHb at a normal Hb iliev,el ,of about 15.4 g/lOO ml.
The hemoglobin co.ntent was not aUered by 'exposure. Thus there was no aocelerated breakdown of hemoglobin with attendant CO production during exposure.
The following supplementary experiment was conducted in order to ascertain whether COHb wa.s formed in the blood.
Air containing 1,740 mg/m 3 of methylene chlor~de was admitted into a 50-rol rotating round retort containing 5 ml of heparinized blood (temp. = 37°C). After 3 h of exposure the COHb level in1Jhe bLood diJrninished. Thus the expertment showed that CO is not formed in the blood under these experimental conditrons. The amount initially found was flushed out to some extent during the course of the experiment. Thus exposure to methylene chloride probably results in the formatio.n of CO in some other organ ,than the blood, e.g., in the liver.

DISCUSSION
As in previous studies no effect on blood circulation or r,espiration could be demonstrated ,as a result of ,exposure to a s'Olvent (1,2,3,4). Thus neither methy.lene chlo-  impairnnent in '1:Ihe present study was ap-parent1Jy due 'to rt1heiiact that the present concentrations wer.e much lower. Thus subj-ects did not display more than about 4 % CORib when work was perlormed at the end of th-e fourth period. The concentration of methylene chloride in alveoilJar an-was relatively high at rest in relation to :the degree of -eX1Posure. The ooncentration ,rose stepwise during ex:erci'se wiJth incr·easing intensity. The arteri-aJ1 blood ,concentra'tion displayed a .rate of increase which deolined, lespeoially during the founth~riod. Similar courses for ,the concentra'non in aliveolar air and arterial blood were obtained in ex:posure to sUlbstances with l~ited 1S01U1biiliity in blood (2,3). Howev-er, 't1he ,alveolar membrane must not impede didJfUSion of the substance from ,air into blood. Van Rees {20) rec,ently poinJted out that 1Jh-e il'lalt-e of diffus~on {)If la solvent through the 'ailveolar membrane is; probab[y never the ,limiting f,acto,r in the uptake ,in blood. 'Dhus methylene chlo!I'ide uptake rn !the org.aniSlm priJInariJy depends u:pon its solubilHty in blood. The resting upta!k.e /fleported lin ' alveoJLi, wihiJch declined in rthe course of time hoth during constant exposure and durintg eX'pQStme wiJ1lh moreasing 'WOI"k intensity, fwlly supports this view. InI1~oJ.'lts on the studies of toluene (1) and methyl1chilorofiomn (4) it was recommended ,that e~osure during oooupartional work should be ,controlled ,by measuring ,the COOlicentJrration 'm ambient atr rand sametiJmes ,also tha:t in aJ1V1eolar air.
Since during e~sure me1JhyJ.rene 'chloride Shows siJrnilaJri1Ji.es to these two solvoots, both methods should also be used in conjunctiron with eXiposure to methylene chlodde.
As noted in the sedian on metabolism, extra CO is formed during .exposure to methyJene ohloride. A small amount of endogenous 00 is ,atlwrays produced, as mrst demonstl'lated hy 'Sjostrand in 1949 Irrespective of its genesis CO blocks oxygen uptake in the organism. This very serious effect of CO has led to extensive discussion of the TLV for CO in air. Scientists at the National Institute for Occupational Safety and Health .in Sweden feel that a reduction to 35 ppm is justified in order to prevent a COHb content exceeding 5 0J0 (26). The U.S. value, Uke the Swedish value (December 1974), is currently 50 ppm. The COHb level, including the resting value, exceeded 5 0 /0 when subjects were exposed to the TLV of methylene chloride (500 ppm or 1,740 mg/m 3 ) £01' 2 h. The concentration would increase further during ·an 8-h work day (19). Therefore, lowering the TLV of methylene chloride to such an extent that the COHb is never able to rise to more than 5 Ofo is jus.tified. The U.S. value was Deduced to 250 ppm in 1972 (5). However, thisreductioill is probably inadequate. As mentioned earlier, additional carbon monoxide corDesponding to 0.5 gllOO ml would be formed during exposure to 250 ppm for 2 h. With an ordinary resting value of slightly more than 0.1 g/100 ml the COHb level would then amount to ·about 4 Ofo. The concentration might then rise to more than 5 Ofo if exposur·e to this concentration continued for 8 h.

METHYLENE CHLORIDE
This circumstance should be sufficient justification for supplementing the aforementioned sampling of ambient and alveolar air with an additional test, viz., COHb in blood. However, it is obvious that any such blood sampling would meet with considerable practical difficulties. Therefore, it is suggested that the TLV for methylene chloride be set at a level which guarantees that the COHb content of blood will never exceed 5 Ofo.