Document 5bZKgO0qJaZw5ooBZLz2099YJ

billonnai-ed hydrocarbons and the environment G. McConnell, D. M. Ferguson, and C. R. Pearson In recent years there has been concern about the potential toxic hazards of chlorinated organic compounds used as insecticides. It has come to be assumed that similar hazards arc presented by chlorinated hvdrocarbuns generally, many of which are manufactured and used in large quantities. The evidence reviewed in this article indicates that while a number of C, and C. chlori nated aliphatic compounds are widely distributed, they do not accumulate to the same extent as the chlorinated insecticides nor do they have their extreme persistence in the environment. In recent years concern lias heen shown over the environmental effects, on both wildlife and man himself, of the organo-chlorine insecticides tof which DDT is a familiar example) and the poivchlorinated biphenyls fPCH). This concern has extended to all 'chlorinated hydrocarbons': consequently they have heen included in the class of chemicals banned by the Oslo and Paris Conventions to regulate the discharge of waste into the sea. We hat e made a study of the potential environ mental impact of those chlorinated aliphatic hydro carbons that are manufactured on a large scale. Several surveys relevant to such a study have been published recently, either as review articles or in government or intergovernmental reports: a useful discussion has been presented by G. T. Goouman [il. The following factors must all be considered, 'a t What quantities art produced; how, where, and in what form the materials are released to the environment; ,b; how they are distributed, and in what concentrations thev occur in the atmosphere, in fresh and marine waters, and in soil sediments; (c) at what levels they exhibit toxic effects to man or wildlife; (d) whether they accumulate in living organisms, and, in particular, Irecome concentrated up food chains; (e) how stable they are in the environment, and whether any of the products of degradation are themselves likely to have ill-effects. Emlaalon and distribution pathways World production capacities for all the major chlorinated hydrocarbons are shown in Table l. Of the compounds in column (a) die fluorochloromethanes are extensively used as aerosol propellants, and are therefore almost completely lost to the atmosphere after use. The others are employed principally as solvents in industrial or . McConnstt. M.A., S-Sc. W*a barn in England m tflt Educated at the Cctlgiai School IWeroool end lh# o< Osiord. whtt# ni graduated in chenvatry m 1941. In 1M2 h* joined tha R*tfcn Oeoartment ol tn man General Chemicals Olvielon of fCI Utnce nw?ed Mood Qivt&toM. h naa been mvowd n range of Indutlnal research Drcolenti, mainly n the field of organic ehemicala end unci 1071 hat had a major imereti in the environmental taiOAct of the hato-orgsnie comgounot. D. U. Ferguton. B Sc. Ph D- Wi bom m Scotland >n 1M0. He omduntvd (in ohy svetogy) fro<n Aberdeen University m 1963 and ohl.uncd h*f doctorate from Edinburgh University in INf. In 1966. he joined ICI't induauml Hygiene Research Lahorelones (now Central Toncology Laboratory! wnere no hat been concerned with the tatjeologicaf evaluation ol a *de range of industrial and agricultural cheaUcaia. C. R. Pearson, M A, Was bom In England m t9J$. Reed Natural Sciences at the University of Cambridge, oci m biocncmutfy. He joined tl m 19S2. .it the Brirham Laboratory >n Ce^on. He h*ii ben involved m a wide range of tudiea associaiad iih both assessment end prevention of waier pollution. domestic applications. Although industrial solvents arc :n a large extent recovered, the production of these sot', mis represents the need to replace the handling losses incut; *d in their use. The quantities in column a t therefurc repre sent the annual losses to the environment. This loss mil arise from a multiplicity of sources, correlated u uh h. aj factors such as population density and industrial actr. .r.. The products in column 'b) serve primarily as chemi cal intermediates. The amount of them that enters the environment is a very small fraction of the total produc tion, and is limited to relatively feu chemical manufac-. curing sites. At these sites smaller amounts of bv-produ'. ;s will also be produced ; these ranee from other chlorinated Cj hydrocarbons to chlorinated hydrocarbons of hi_:: cr molecular weight and tarry residues. These residues have in the past been dumped partly on land, but panicula: i\ at sea; A. Jerneiov and co-workcrs [a] ha\e describ'd the effects of`EDC tar' dumped in the North Sea. 1 be residues are now either incinerated under comrolicd conditions, or returned where possible as raw material to modified production planes, in particular for manu facture of carbon tetrachloride. The products listed in Table i are characterized bv high volatility and low solubility in water; some ol' their physical properties are show n in Table 2. They enter the environment primarily by evaporation to the atmosphere. Some chlorinated hydrocarbons will, hem ever. be found m aqueous effluents from factories handling them, and even in household sewage, and so w ill pass into municipal drainage systems and rivers. During the determination of rhe partition coefficients shown in the last column ol Table 2, it was found that there is a rapid transfer of t1 .c chlorinated compound both from air to water, and from water to air. Irrespective of whether the initial loss '(' chlorinated hydrocarbon is to the atmosohcrc or :l r hydrosphere, the transfer processes will lead to a w.uc distribution of these compounds, with .serial transport playing a major part. 5u<*h processes explain the occur rence of some chlorinated hydrocarbon in rainfall and upland waters. Table 1 Estimated world production capacities (1973) of major chlorinated hydrocarbons, in 10* lon% year (a) (b> TrichloroathyJari* Perchloroelhlyene t.1.1-Trichloroeihan 1010 1050 <80 Molhylcne chloride 400 Tnc hforo floor om ethane <95 Diehlorodifluoromcthane 570 Vinyl chloride 1.2-Oichforoeihane Carbon tetrachloride Chloroform Methyl chloride 10 500 13 500 1000 2<$ 350 DSW 0 2 8 0 1 5 STLCOPC Occurrence in the environment 'T rrhniqmi M'T the I*1 Ti tit>n. idemiHeat inn. and measure ment <S 1 )t) \ and Pi `11 ln\ c been exteusiv cly studied and dr-mbed. A!un*st all depend on solvent extraction, careful dc.in-Hp by column chromatography, and measurement by gas-liquid chrnmatogrnphy (GLC) U'ina nu electron capture detector. Where possible, tetrachloride :uu widely distributed at the Mg,kg level or lower. In addition sonic samples of sea water from Liverpool liay. w liich rccciv m l.u ^c > olumcs uf industrial and domestic ctllncms, contain traces cless than 1 ug/kg) of tctraclvlorr'ctlinnc. prmacblniuclh.uie, pcut.v and hcvaclilofobutadicuc iHGJJD,, hcxachlorobenzcne, and y-bciucnc hcxachloridc. identification n ronfii mud bv a linked mass-spectrometer MS . Altlunich rite s.nnc basic methods can be applied Samples of marine sediment from Liverpool Bay con tained the same compounds as the ovrrlving waters, but to the oihrr tmhiMitnlU important chlorinated hvdroenrbons, then hi?h volatility introduces considerable there was no correlation between the chlom-organic content uf the sediment and that of the mid-depth water d i micu it ir? at the sampling, extraction, and GLC from tlie same point on the sampling grid. It appears that , separation sragev. In addition, the response oTrhe electron coarse gravels have (title adsorptive capacitv for these capture detector vanes wicirlv according to the number compounds, whereas sediments rich in organic detritus of chlorine atoms in the molecule. and this results in very have a much higher adsorptive capacity. Averaged over considerable differences in ihe sensitivity and accuracv all samples, the concentrations of chlorinated C1('C2 of determination* o! different compounds. But these compounds.in the sediments were similar to those in-the di.'ncuhir' have been overcome. . water; hcxnchlorobutadiene with its higher molecular Initial observations of the occurrence of chlorinated weight does, however, show concentration factors up to hvdrocatbons in the atmosphere have been made by a hundredfold. J. P. Rilry (3]. J E. Lovelock [4], P. . Wilkniss [5], and P. G. Simmonds [6]. Our own observations in the Occurrence In animal tissue atmosphere of north-western England, as well as a much Details of the distribution of chlorinated hydrocarbons fuller range of measurements in water, sediments, and in the tissues of marine plants and animals are being marine organisms aie given elsew here [7]. All the authors published [7]. The compounds found in sea water are agree that fluorochloromeihane, tri- and perchloro- also found in marine animals. Concentrations vary from eihylene, chloroform, and carbon tetrachloride are found 1 Ug kg or less in invertebrates, to 10 ug,kg in the flesh universally in the atmosphere at concentrations normally of fish, to a maximum of 50 pg kg in the eggs of sea birds in the range betv'een 1 and to ng I; we ourselves also and the blubber of seals. Special attention was paid to found t. \, i-trichluroethane ax a similar concentration. the fatty tissues of sea birds and mammals, as these The distribution of fiuorochloromethanes and of tri- represent the highest trnphir levels in which DDT and chlororthv lene. prrchlorocthylene. and trichloroethane, which are major solvents, is consistent with their usage. PCB are found to accumulate. As man occupies a similar position at the head of a series of food chains, The concentrations of chloroform and carbon tetra analyses of a wide range of foods, of both animal and chloride. which are chemical intermediates with limited vegetable origin, have been made: the results are shown losses, is somewhat surprising, and led Lovelock to in Table 3. Finally, it has been possible to carry out suggest that they mav arise naturally by reaction between some analvses of post-mortem human tissue; results are chlorine and methane in the atmosphere. Analyses of rain-water, rivers, municipal water sup presented in Table 4. During the early stages of these investigations it was not possible to distinguish analyti plies, and the sea (7] again show that tri- and per- cally between carbon tetrachloride and trichloroethane, cnloroethylene, trichloroethane, chloroform, and carbon but improved technique allowed this later. A summary of the results from this and the previous section is presented Table 2 P^ysia! orocerhes of so'ne tfiphtiic chiorinaftd hydroetrbons in Table 5. From these results we draw- two main conclusions: .'a) The b.p. ro Vethyl ehlorlds M#thyln* ehlorida Chloroform Carbon Utrachlorlda Ethylan# dichtoride 1,1,1-TricMoroethan* Vinyl chloride Vinyhden* chloride Trichloroethylene Parchloroethylcne Fiuorotnchlof ometnah* Difluoriict icromeihane Heiacr.iorobuijcene -24-2 40 1 3 76 1 Ul 74 1 --13 9 31 87-0 1212 -2923-8 8 213 Vapour pressure ' (mm Hg,'20*C) Solubility in water at 20*C (partsMO* w/w) Partition coefficient water'air at 20C (w/v per w/v) 3788 362-4 iso-s 00-0 63-9 96*0 2320 496-5 579 14-0 667-4 4306 0-15 72SO13 200 (23*0 8300 7*3 >600 460 ftoercy 400 1100 fM 1160 220 (25T *2 13-3 8 86 11 26-4 0-71 MB (10'C) <ut 2-74 1-22 0-03 0-06 097 concentrations of DDT and PCB in fat tv tissues are at least three orders of magnitude greater than those of the industrial solvents, (b) Chloro form and carbon tetrachloride are very widely distributed, and at un expectedly high concentrations. PotsIbJo hazard to human health The ubiquitous distribution of the chlorinated hydrocarbons means that a population may breathe air, drink water, and crmv.imc food containing trace quantities of these m.iicriala. In many instances, the cooking of a foodstuff will reduce these level* still further. There is. moreover, evidence for the presence 0! these compounds in human tissue, again at extremely # Unctr 760 mm pietsure ol orcanoentorme compound iow concentrations (Table 4). 14 Table 5, vvhieli summarizes all the analytical data, demonstrates that there is no evidence fur significant accumulation in human tissue of any of the materials under discussion. Rather, they indicate a general back* gTOund, at the parts in 10* level, which pervades the whole ecosphcrc--atmosphere, hydrosphere, and bio sphere. These results are in sharp contrast to those reported for DDT or PC-B, where accumulation in tissues of several orders of magnitude has been observed [8]- ' It is interesting to compare in Table 6 the threshold limit values 1TLV1 set by the American Conference of Governmental and Industrial Hygienists with the maxi mum atmospheric concentrations measured in our survey [7]. The threshold limit values define permissible atmo spheric concentrations applicable only to an 8-hour working day and a 5-day week. In the United Kingdom, maximum allowable ground level concentrations to which the public might continuously be exposed are set in the range of 1 /25th-- t /40th of the TLV. It can be seen that the environmental concentrations are less by some Orders of magnitude. If we examine the possibility of poisoning by swallow ing, we find front tests on rats and mice f 10 -18 J that a!! five aliphatic compounds of Tubic b hate lou tn\,cities with LD50 values varving bets'ecu 2 and i.-.- kc. The LD50 value is the dr.se that "ill kill 30 per cent of a group of experimental animals.) The corresponding LD50 value for DDT "ith rats "as oiy-o-g a kg. Clearly the presence of 1 few parts per to3 of these substances in food or water will not give rise to acute poisoning. The acceptable daily intakes ADI', of jubilances in foodstuifs are set by bodies such as the FAO Ur.,1. r.c Party of Experts and the WHO Expert Uruup nn Pesticide Residues. They are generally calculated b' applying safety factors (of 100-20001 to the maximum no-elTect levels observed in long-term feeding studies t. r. animals, taking into account the absorption, metabol.srn. and excretion of the substance. Absorption, mstabollsm, and excretion Published reports indicate that most of the chemical, are moderately to ..ell absorbed from the gastrointestinal tract after oral administration (20, at, 22]. Tvpicalb, a very large fraction of the administered dose is excreted unchanged in the expired air. It has been established Tab)* 3 Chlorinaitd hydrocarbons in foodstuifs (Concantralions in uj, kg) Foodstuff Chloroform Dairy product Froth milk Choshiro chooso English butter Hant toot Moot English boat (sisek) English boof (Ut) Plg't livor Oils and fats Mtrgtrln* Olive oil (Spanish) Cod livor oil Vtgolablo cooking oil Castor oil Battragt a Cannsd fruit drink Light sle Cannod orango juict InsUnl coftoo Tea (packet) Wlno (Yugotliv) Fruit and vtgtiablas PoUtoot IS. Wilts} Potatoes (N. W. England) Apples Peers Tomatoes* 3 33 22 14 4 3 1, 3 IQ 2 NA 2 9-4 1 t 1* NA 19 4 9 * > Carbon tetrachloride 02 5 14 0-9 7 a 9 6 11 19 0-7 NA B-S 0-2 5 4 0-3 1 3 s < 45 Trichloroathant Trichlorotthylens Psrchlorosthylant 0-3 03 32 10 13 09 ND 3 IS 0-9 < 12 1-0 4 22 5 a7 10 t 7 5 19 2 7 0 01 9 ND 3 52 0-7 ND NO ND 43 7 60 3 0-02 ND 4 ND ND 1 3 0-7 3 52 24i 1*7 1*2 Block grapes (imported) f Frtsh brtad NO > 19-7 3I 21 ND 71 * Tomato plants were grown on a reclaimed lagoon st Runcorn Works of ICU f HCBD is still used in some countries as an insecticide for vineyards. I TCE, PCE -- Tetro-, pcnlachiorovthano: HCBO HosachloraOutadienv; HCB ^ Hesschlorobentene NA -- No Analysis; NO -- Not Detected Others HCBD 0 08 HCB 1 ND HCBD 2 ND NO NO TCE 0 3 PCE 0 4 PCE 0 8 ND TCE 2 HCBD 03 HCB 0 7 NA PCE 0*6 HCBD 0 2 ND ND ND NA NL TCE 0-7 ND ND HCBO 06 HCB >01 TCE ) 0 HCBO 3 7 ND OSW 0 2 8 0 1 7 STLCOPC Teble 4 Occurrence of chlorinated hydrocarbons in human (issue. Concen(ri(tcni m vp.fcy (*et tissue) Age of subject Sex Tlssu a 78 F Body fat Kidney Liver Brain 78 F Body fat Kidney Liver Brain 82 F Body fat Liver 48 M Body fat Liver 65 M Body fat Liver 75 M Body fal Liver 66 M Body fat 74 F Body fat Chloroform Carbon tetra chloride + frichloro' ethane Trichloro ethylene 19 2 5 4 9 5 t 2 67 8-7 67 9-5 . 64 8-8 65 too 63 52 24 1 1 <1 4 3 A t 1-6 33 21 . 35 11-0 3-5 13 6 5`1 1-6 52 52 <1 3 1 2 3 2 ci M 32 6-4 35 34 5-2 14-1 38 46 49 Perchloroethylene 6 <0-5 <0-5 <05 1 6 2 <5 0 4. 12 08 0-7 21 3-4 292 43 08 4 HCBD _ -- _ -- -- 0-8 12-1 18 3-7 1-8 11-8 0-8 13-7 1-2 4 Others ,_ _ ___ _. _ _ _ -- TCE <05 y-BHC 24 y-BHC 7 5 r-0Hc 22 that metabolism results in the formation of trichloroacetic acid (from trichloroethylene and perchloroethylenc-, trichloroetlianol [from trichloroethylene), chloroform (from carbon tetrachloride), and CO, (from carbon tetrachloride and chloroform) [20-53]. Some of the compounds have non-industrial uses and it is possible to compare permitted use concentrations with the maximum concentrations found in the survey described above. The relevant data are summarized in Table 7. This table demonstrates that, with one exception, there are enormous differences between permitted-use concentrations of the chemicals, admittedly in some instances for discontinuous use, and the maximum levels found in food. The exception is carbon tetrachloride, \\ here the suggested guidelines for grain residues are similar to the amount of this material in food; the grain TsbltJ Occurrance of chforinatad hydrocarbon* in tha anetnnmant. Typical concentrations (w w) of the five major compounda (chloroform, carpon latrachfonda, inchloroaUtylana. perchloroathylena, tnchloroathana) Air Rain water Surface water Potable water Sa *tf Marin# sediments Marine invertebrates Fish Waterbirds Marine mammals Petty foods Non-fatty foods Human organ* Human body fat Minimum Maximum 10"* 10" 10-" 10-* 10-" to-* 10-" 10-- o-" 10-* 10-" 10** to-* 11J-* 10-* 10-* Id** >10-* to-* 10-* w 1" * 10" 10" 10-' to-* 19 residues, however, do refer to regular daily intake. To put the remaining figures in perspective, some 500 kg of cheese would have to be consumed to give an intake of chloroform equivalent to one dose of the linctus described. CfTaets on wlldlifs So far only a few results are available for the levels of the Cj/C, chlorinated hydrocarbons in land animals (other than marine and avian species) or plants. The indica tions are, however, that the levels in such species are not significantly different from the general background con centrations shown in Table 5. Hexachlorobenzene (HCB) and hexachlorobutadiene (HCBD), of which we have found traces in fish and foodstuff samples, are used on a small scale as pesticides. J. G. Vos [24] showed that toxic effects occur only in birds fed high dose rates nf HCB, namely 20 mg. l over 3 months. At the end of the trials, the average HCB concentration found in the liver was 35 mg/1. Our own study of effects on marine organisms has been described [']. Acute toxicity levels for marine fish, barnacle nauplii, and a unicellular alga are reproduced Tsbio 6 Threshold limit raiug compered with maximum observed concentrations (mg.l) Trichloroethylene PercHforoethyfene Tr>ttilw4lhn Cirbc-n tetrachloride Chloroform TLV 0-50 0-87 1-00 <H* 0-12 Max. observed concentration 0 00013 0-00009 0 000020-00000* 0-00004 9 Not dflerer.tiat*d by analysts. therefore, the concentration ihown ft the maximum value, assuming the GlC paak represents wholly tnchloroetn&ne or carbon tetrachloride. DSW 0 2 8 0 1 8 Teble 7 Comparison of observed concentrations in food end permuted use concentrations Chemical Uses Trichloroethylene Perchloroethytene 1,1,1-Trlchloroethane Anaesthetic Extraction solvent Antihelminthic (veterinary and human medicine) ? Carbon tetrachloride Grain fumigant Chloroform Anaesthetic Flavouring agent US food and Drug Administration (1973) food Chemical News Guide * t Msrtlndele 'The Extra Pharmacopoeia' 36th Ed. (1972) i FAO/WHO Expert Committees, WHO Pesticide Residues Serios I, 1971 (1972) Permitted concentrations/doia 10-25 mg/Vg decaflfinsttd coflee* 1-6-8 0 g/50 kg r (therapeutic dose) ? 50 ug/kg - (cooked cereal products): 15-30 mg,dose t (cough iinctus) Me*, concn, measured to (picket coflte) 13 (butler) 20 (black grapes) 20 (black grapes) 33 (cheese) in Table 8; comparison with Tabic 5 shows that they are It has been suggested that low concentrations nf at least three orders of magnitude higher than chose chlorinated solvents could, however, inhibit mi'-rnbi.i! found in sea water. There is no evidence that the bio degradation processes generally, especially in anaerobic accumulation up food chains, which is such a feature of environments. Particular concern has been e.vpie,-d DDT and PCB, occurs to any significant extent with the over the effects on sewage treatment. Our observation; commercial solvents. There is some indication that in show that concentrations in raw sewage are noir.i.i'A- fish, by-product 'heavies' such as HCBD show some less than o-t ing,l; some of this is adsorbed on to priinarv bioaccumulation intermediate between the two former sludge, while most is lost to the atmosphere during classes, but this does not increase in birds and mammals biological oxidation, either by activated sludge or feeding on fish. trickling filters. Our initial experiments show that inhibition of aerobic Degradation In tha environment oxidation does not occur at concentrations lielow 10 mg 1; Biochemical degradation. As the major chlorinated hydro this concentration will not normally be found in a.nv carbons are so widely distributed in the environment, sewage works. A11 extensive investigation of the effect.; they are exposed to a wide range of potential degradation on anaerobic digestion in sewage ivories, particulars by routes. Although there is evidence that they can be chloroform, is given in {25]. metabolized by mammalian tissues, we do not know how Physico-chemical degradation. There is evidence for the many other phyla have this ability. It is generally chemical breakdown of chlorinated hydrocarbons in accepted, however, that micro-organisms, whether water, but in general these reactions are very slow : we aerobic or anaerobic, do not have it. Microbiological estimate the chemical half-life of perchlornetiiylcne in degradation, which is >0 important for the destruction of water to be about six years. Exceptions to this aide for many organic compounds, therefore probably does not compounds which are easily dehvdrociilorin.ncd: the have any significant direct part to play in this case. reaction rate in such cases depends on the pH. An In mammals, the metabolic pathways of all the com important exampte of this is the dchvdioclilorinatian nf pounds discussed so far lead to chlorinated acetic acids, the solvent 1,1,1-trichloroethane, which would base a either directly or via chlorethanols. Chlorinated acetic chemical half-life in sea water ;pH 8. to:Cj of about n acids have all been shown to be susceptible to further months. The decomposition product is vinslidene degradation by micro-organisms in sea water [7]. chloride, with (at environmental temperatures oniv a minor amount of acetic acid arising Tabl* by hydrolysis. Rapid degradation in Acute toeIcily of chlorinated hydrocarbons to marine organisms (Concentrations eapressed ss mg I) aqueous sy stems does, hots ever, occur ' in the presence of metallic iron. At 6 h LC ae to Dab (flat-fifth) 4ft h LC it to Barnaefe ntupiil EC it to Unicellular algae the present time the resulting degra dation products have not been iden tified, but this faster rate in the pre Trichloroethylene Perchloroethytene Triehloroathana Chloroform Carbon tetrachloride IS S * -90 10 s 14 -- -- 1 sence of metals could be important. 10-9 The current implication of our 9 work is that it is by tropospheric -- photo-oxidalinti that liiccus irunmen- -- ta! aliphatic otgnnn-chloiinc com Heaachlorobutadiene Ethylene dlchlortde Propylene dichloride *49 1t9 *7 H> 109 9) pounds are principally dcstrused. UO Thil will apply also to those com to pounds present in the hydrosphere, 17 DSW 0280L9 STLCOP< since transfer reactions bct'vccn hydrosphere and atmo sphere tirrtir rapidly ('Gl- Oxidation can easily Itc demon strated by introducing a few mij/l of the nrg.mor.hlorinc compound into a scaled quartz llask and exposing the latter out of doors. Monitoring the organic residue shows that the mean half-lives of the chlorine-substituted cthylenes lie in the range &-12 weeks, and of the chlorinesubstituted methanes and ethanes in the range 10-33 weeks. Thrse experiments are naturally influenced by diurnal and climatic s anations of temperature and inci dent solar radiation, and the half-lives are reproducible only to within =50 per cent. N'nnetheless, they serve to shots that tiie simpler aliphatic organoclilorines do not have the high persistence associated with the chlorinated insecticides and PCB, nor on the other hand are they sulficiently reactive to give rise to photo-chemical smog, a view confirmed by the work of M. F. Brunelle it al. [27]. Similar experiments have been carried out exposing the flask to radiation front a xenon arc, suitably filtered to remot e radiation below apo nm, taken as the lower limit of tropospheric solar.radiation. Under these condi tions of constant radiation flux and temperature it has been found in a number of cases that (lie order of the reaction with respect to the halo-organic compounds is fractional, tending to zero at the higher (too mg/I) initial conceritrations and to unity at the lower concen trations (o-t mg/1). At the much lower concentrations (ng I) found in the troposphere we can--at least to a first approximation--assume a first-order reaction with respect to the halo-organic compounds. The kinetic pattern observed does not appear to arise from heterogeneous reactions, since packing the reaction lire.vessel with quartz wool has a negligible effect on half It can, however, be explained in terms of initial attack on the halo-organic compounds by some transient species present in the troposphere, possibly by active photolysis products of such species as nitrogen peroxide, ozone, or chlorine, known to be present in the tropo sphere at concentrations very similar to those of the halo-organic compounds. Deliberate addition of these inorganic species in the xenon arc 'experiments can accelerate the decay rate, even when the inorganic material is present in much less than stoichiometric equivalence. The implication is that, because of the excess of halo-organic compounds over trace inorganic species present in the outdoor flask experiments, the latter do not rcpicsent a true microcosm of the tropo sphere. In that case, the half-life ranges quoted above are probahly over-estimated. U e have also identified the degradation products arising from xenon arc exposure. For the most part these are simple inorganic species i.CO, CO;, ll.O, HCI) already present in the atmosphere: in some cases (CCI,t CHjGCIj) molecular chlorine can also be formed. Only tri- and pcicliloioethvlcnc yield relatively stable chloroorgaruc intermediates, the di- and trichloroaceiy! chlorides, w ith minor amounts of phosgene. The latter rapidly hydrolyses to CO- and HCI, but the chloroacetvl chlorides will enter the hydrosphere as the di- and trichloroacetate anions, which have very long chemical half-lives at environmental temperatures. However, we have already noted the microbial breakdown in sea water of both these anions, complete oxidation occurring during a 20-day incubation period. It thus appears that not only are the simple chloroaliphatic compounds not particularly persistent, but their degradation products are simple species commonly found in the environment. Conclusion From evidence so far available we conclude that several chloroderivativesofntethane.e thane, and ethylene are very widely distributed in the environment at a level of concen tration of the order of 1 part in to*. The particular com pounds are chloroform, carbon tetrachloride, trichloro ethylene, perchloroethylene, and irichloroethanc. The as sumption that the last three occur as a result of chemical manufacturing processes is tenable, but the occurrence of the first two may also be due to unidentified geochemical processes. All these compounds are fairly rapidly degraded in the environment to carbon dioxide, water, and chlor ide ion, and there is no evidence for their significant bio accumulation, via the food chains, to higher trophic levels. R*frnc*i [ij Goodman, G. T. Pro*. p. So*. 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