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Technical aspects of gaseous formaldehyde as a sterilant
Technicalaspects of gaseous formaldehydeas a sterilant Vagn Handlos Royal Danish School of Pharmacy, Depanment of Pharmaceutics, (Received 13 March 7983; accepted 28 April 1983)
The design of a sterilizer for sterilization
Universitetsparken
2, DK-2 100 Copenhagen,
of heat sensitive items using gaseous formaldehyde
cribed. The sterilizer is able to create a constant formaldehyde-steam hour. It is further able to operate at formaldehyde
concentrations
concentration
close to saturation with only small residues on of formaldehyde
poly(vinyl chloride) being approx 15 and 250 ppm, respectively,
at 70°C and 30 mg WHO
of formaldehyde
and steam is des-
over a period of at least one
sterilized plastic materials. The autoclave is used for measurement coefficient
Denmark
solubility in polyolefines and f’.
The diffusion
chloride) is measured and is in the same order of magnitude as for ethylene oxide in poly(methyl methacrylate) ie. 10-l” cd 2’. M easurements of airborne formaldehyde in front of different sterilizers
Keywords:
in poly(vinyl
show that it is possible to design facilities
Sterilization,
formaldehyde,
diffusion
1984
Euttemvorth
Et Co (Publishers)
exposure limits.
coefficient
Due to drawbacks arising from the usage of ethylene oxide as a sterilant a growing interest has been registered for the usage of formaldehyde as a substitute”2. Formaldehyde at high relative humidity has been used for sterilization of heat sensitive items in British and Scandinavian hospitals for many years. The use of the method has been limited, by the uncertainty as to its microbiological efficiency which has been tested only to a limited extent under circumstances which can be reproduced. There is still uncertainty on the toxicological aspect of the method arising from gas residual on sterilized devices and exposure of the personnel to airborne formaldehyde. The lack of reproducibility is due to the fact that gaseous formaldehyde, in mixtures with steam or alone, is a complex physicaCchemical system due to its very high water solubility and its ability to polymerize3. This is the reason why the gas phase concentrations in the autoclaves, calculated on the basis of the amount of formalin added, are very seldom in agreement with the actual measured gas phase concentrations. Published results3-5 on measurements of gaseous formaldehyde in sterilizers show that the concentration of formaldehyde even in the unloaded autoclave decreases in the course of time. This indicates that formaldehyde evaporated in the autoclave is removed either by polymerization which takes place on the cold parts of the autoclave or by dissolution of formaldehyde in steam condensates. Measurements of formaldehyde gas phase concentrations in an autoclave during sterilization are thus a useful method for control of both the physical and the chemical conditions in the autoclave. The toxicological evaluation of formaldehyde as a sterilant is divided in two parts, one dealing with the 0
meeting occupational
exposure to airborne formaldehyde of the personnel and the other dealing with formaldehyde residues in the sterilized goods. Occupational exposure limits vary from country to country, but are approximately 1 ppm or identical with the olfactory limit. No official limits for formaldehyde residues in sterilized goods have been published. The purpose of the present investigation has been to design an autoclave which unloaded can keep a constant formaldehyde gas phase concentration during sterilization, to supplement previous measurements of residual formaldehyde in thermoplastics and to measure airborne formaldehyde.
EXPERIMENTAL Apparatus
PROCEDURE
and chemicals
The autoclave was made by Getingeverken, Sweden, for experimental use only. Chamber and door, fully jacketed, were made of stainless steel, whereas pipes and fittings were made of copper and brass. The cylindrical chamber (100 I) is fitted with a vaulted bottom in one end and a door in the other. The longitudinal axis of the chamber is tilted against the horizontal plane to facilitate the catching of condensate in the bottom valve, which is built into the heated jacket to prevent cold spots. The steam and formaldehyde leading pipes are equipped either with a steam-heated jacket (1 10°C) or are built into the water jacket of the chamber. Steam generator and water jacket are both electrically heated. The functions of the autoclave (cycles, pressure, temperature, etc.) are programmed to operate automatically. Pressure and temperature
Ltd. 0142-9612/84/050081-05$03.00 Biomaterials
1984,
Vol 5 March
81
Formaldehyde
sterilization:
K Handlos
3 mmHg and & 0.5”C) in the autoclave are registered continuously. The gas chromatograph was a PYE model 44 provided with a hot wire detector and operated under the following conditions: columns bore 4 mm, length 1.5 m, packed with Porapack T; carrier gas nitrogen, flow rate 35 ml min.-‘; column temperature 120°C; detector temperature 150°C; detector current 11 OmA. The autoclave was connected to the gas chromatograph by means of a l/l 6” stainless steel pipe transmitting the gas direct from the autoclave chamber to a gas sampling valve, from where a sample of 1.81 ml was injected onto the column. The pipe connecting the autoclave and the gas chromatograph had a 0.5 m free flexible end for sampling in the chamber. Throughout its length the pipe was heated to minimum 110°C by an 8 A current. The sampling loop and the gas valve were immersed in a silicone oil thermostat of 130+l”c. The sample oxidizer was a Packard Model 3306 Tricarb and the scintillation counter a Beckmann LS 200. The formaldehyde was used as a non-methanol stabilized formalin containing 37.4% HCHO; the “C-labelled formaldehyde as a 1.3% aqueous solution, specific activity 1 .13 mCi/M HCHO from Amersham Int. The scintillation liquids were Pica-fluor 15 from Packard and Lipoluma from Lumac. The composition of the plastic materials is given in the references6. (k
Methods The autoclave is operated at a jacket temperature of 70°C and all cycles start with three steam pulsations between 50 and 245 mmHg for air removal. During the subsequent vacuum the formalin is injected into the steam heated U-pipe. The autoclave is then filled with steam to the pressure of saturated steam plus the partial pressure of the added formaldehyde calculated as an ideal gas. The sterilization phase is followed by a cleaning of the autoclave by 6 steam pulsations varying between lOO245 mmHg. The gas chromatographic determination of formaldehyde and water vapour in the autoclave atmosphere is made using helium as internal standard, and 250 ml of helium (2O’C. 760 mmHg) is added with the formalin. Standard deviation of the gas chromatographic determination of formaldehyde is 2% at a concentration of 30 mg HCHO I-‘. Calibration of the gas chromatograph was carried out with known amounts of formaldehyde in aqueous solution, detector response is determined as peak areas. Determination of the formaldehyde concentration in a hospital autoclave was made by attachment of a pipe furnished with a valve at each end to the top of the autoclave. The pipe was heated to above 80°C (jacket temperature of the autoclave) and the free end connected to a vacuum pump drawing a small amount of the autoclave atmosphere through the pipe. By closing the valves a welldefined amount of autoclave atmosphere was trapped and made available for analysis. Determination of airborne formaldehyde in front of the autoclaves is carried out by the impinger method as described by NIOSH’. The sampling period was 20 min. starting just after termination of the sterilization cycle and opening of the autoclave door. Samples were collected just in front of the door. Saturation of the polymer samples with partially
82
8iomaterial.s
1984,
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“C-labelled formaldehyde was made by addition of 20 $i labelled HCHO to IO ml of formalin. After injection into the autoclave this mixture gave a gas phase concentration of 30 mg HCHO I-‘. In this steam-formaldehyde atmosphere polyolefine and poly(vinyl chloride) samples were saturated with formaldehyde during a 6 h exposure. After the treatment the PVC samples were dissolved in dimethyl formamide (0.7 g in 10 ml) and 1.5 ml of this solution and 2 ml of scintillation liquid were counted for 14C decay. The polyolefine samples (70 mg) were burned in the Packard sample oxidizer and the amount of 14Clabelled carbon dioxide was counted after scintillation liquid addition. All results were adjusted for counting efficiency and retrieval adopted by standards.
RESULTS Measurement of formaldehyde gas phase concentrations in an unloaded hospital autoclave daily operating according to Scandinavian sterilization standards shows a quick decrease in formaldehyde gas phase concentration as a function of time after addition of formalin (Figure 7). The autoclave runs at 80°C and formalin is added corresponding to 60 mg HCHO I’-’ autoclave volume. Figure 7 shows the variations between two identical empty runs and that after 25 min. the formaldehyde concentration is less than 1% of the expected value. This finding is in agreement with previously published results3. Despite the low formaldehyde concentration in the gas phase the autoclave is capable of sterilizing goods on a routine basis. Measurements of the temperature in the chamber of the experimental autoclave show that all parts of the chamber have a temperature within 0.5”C of the set temperature. All pipes connected to the chamber are heated to the temperature of the chamber orto 110°C except for a 30 mm %” pipe which is heated to only 40°C. Pressure measurements in the chamber show a leakage corresponding to maximum 4 mmHg h-’ at 180 mmHg. The measurement of formaldehyde gas phase concentration after evaporation of 10 ml of formalin in the autoclave shows (Table 7) that the gas phase concentration is constant during the first hour. Moreover, the measurements show no concentration gradient between bottom and top of the autoclave. The measurements were carried out in three different runs, the sampling tube being placed at different positions as the tube cannot be moved once the autoclave is closed. The results for independent
15
I
Figure 1 autoclave
Formaldehyde
gas
phase
concentration
in
a hospital
Formaldehyde
Table 1
Gas phase
concentration
of formaldehyde
as a function
of
Gas phase concentration top
0 10 20 30 40 50 60
<
0.1 30.0 30.2 30.7 30.0 30.0 31.3
Solubility
of formaldehyde
in plastics
V: Handlos
at 70°C
and 30 mg
HCHO r’
time after addition of 10 ml of formalin Time/min.
Table 2
steriltzation:
mg/l
centre
bottom
<
<
0.1 31 .o 30.0 30.7 31.7 31.7 30.0
0.1 30.7 30.7 30.7 29.8 30.0 30.7
runs are reproducible, but show a gas phase concentration which is 25% smaller than expected according to the amount of formalin added (40 mg HCHO I-‘). The results in Table 7 are not affected by an air intake equivalent to 100 mmHg. The gas phase concentration of formaldehyde after addition of formalin in portions of 2 ml until saturation with steam is illustrated in Figure 2, where these values are compared with calculated gas phase concentrations based on the added amount After addition of 18 ml of formalin the autoclave is saturated with steam; the water vapour pressure in the autoclave was approx. 90 mmHg before the formalin intake. Addition of more formalin gives varying gas phase results due to mist formation in the autoclave. The constant formaldehyde concentration in the gas phase was utilized to expose 0.2-0.4 mm thick plastic films [polyethylene (PE), polypropylene (PP) and poly(vinyl chloride) (PVC) with content of plasticizer from 30-60 parts per 100 parts of polymer] to “C-labelled formaldehyde. After treatment for 6 h the samples were saturated with formaldehyde as longer treatment did not increase the uptake. After the 6 h period the gas phase concentration of formaldehyde was 90% of the initial value. After removal from the autoclave the samples were washed with 0.2 M sodium hydroxide before the formaldehyde assay to remove anypara-formaldehyde from the surface. Noformaldehyde was detected in the alkaline water extract (< 0.1 pg cm-‘). Table 2 shows the formaldehyde content of the saturated samples as well as the counts in counts per minute (CPM); the background for the oxidized samples was 16 CPM and 11 CPM for dissolved samples. Standard deviations for counts on polyolefines were 15% and for the PVC samples 3%.
LD- PE MD-P-t HD-PE PP PVC
16 12 8 24 235
(PgLglg)
CPM 18 12 9 27 279
DISCUSSION Gas phase measurements The gas phase measurements in Table 1 and Figure 2 show that it is possible to construct an autoclave for practical purposes with an almost saturated gas phase concentration being reproducible and constant over a period of time essentially longer than the normal sterilization period (30 min.). This has previously not been the case, as the literatures5 and the examined hospital autoclave indicate. This result is achieved by heating the surface of the autoclave chamber, pipes and valves in contact with steam and formaldehyde. One single pipe was not heated, which did not seem to influence the result This stresses the possibility of constructing equipment for practical use. In spite of this progress it is still not possible to get a 100% retrieval of the added formaldehyde in the chamber atmosphere. The retrieval is approx. 75% at 30 mg HCHO I-‘. It has previously been stated3 that the reason for the low retrieval of formaldehyde in the gas phase was due to dissolution
of monomeric In the actual
concentration mechanisms centration
formaldehyde
and polymerization
autoclave. in the experimental of 2 ml. Solid line:
Solubility
The formaldehyde concentration in plasticized PVC was measured at 27 and 146 h after saturation using the radio chemical method, the samples were kept at 22°C in formaldehyde free surroundings. The value after 27 h was 24 ppm and after 146 h 9 ppm for 0.2 mm thick PVC 30 (30 g plasticizer per 100 g polymer). The corresponding values for PVC 60 were 42 and 12 ppm for a 0.34 mm thick film. If the diffusion coefficient is calculated on the basis of a straight line aeration course in a semilogarithmic plot of concentration versus time, the resulting coefficients are 1 X IO-” cm2 s-’ for PVC 30 and 3 X 1 O--” cm2 s-’ for PVC 60. Measurement of airborne formaldehyde in front of the unloaded experimental autoclave showed variations between 0.7 and 1.2 ppm (~1 HCHO I-’ air). Loaded with e.g. a lab coat (610 g cotton) the variations were between 2.5 and 6.2 ppm and loaded with 1 kg of disposable polyolefine syringes the concentrations varied between 1.8 and 2.7 ppm. Four hospital autoclaves, ranging in size from 20 I to 5 m3 fell into two groups according to their formaldehyde release. Three of them showed concentrations from 2.5 to 3.5 ppm both in loaded and unloaded conditions. One autoclave, volume 300 I, varied between 0.03 ppm in unloaded condition and 0.9 ppm when loaded with four lab coats.
densate
Figure 2 Formaldehyde gas phase concentration autoclave after addition of formalin in quantities calculated values
Material
rules would
case
out cause
as a function
in the steam
in the non-hebted the constant
these
of time.
Riomaterials
formaldehyde
explanations,
a decreasing
gas
An inadequate
1984,
con-
parts of the as
both
phase
con-
release of
Vol 5 March
83
Formaldehyde
sterilization:
K Handlos
formaldehyde from the formalin added would give an increasing formaldehyde concentration as a function of time, as the non-evaporated formalin would be in contact with surfaces above 70°C. At this temperature the concentration of saturated formaldehyde vapour overformalin orpara-formaldehyde is from 50-60 mg HCHO I-’ (Refere”ce 3). Thus the autoclave is unsaturated with formaldehyde in the present experiment. It is most likely that the disagreement between measured and calculated gas phase concentration is due to adsorption of monomeric formaldehyde to the metal parts of the autoclave. Figure 2 shows that the retrieved part of formaldehyde falls from approx. 95% at 8 mg HCHO I-’ to 75% at 30 mg HCHO I-‘. This indicates that formaldehyde is adsorbed onto the wall of the autoclave in competition with steam as the percentage content of formaldehyde in the gas rises with increasing amount of formalin. The gas initially is pure steam. The explanation of adsorption implies that by 30 mg of HCHO I-’ approx., 80 w HCHO cm-* is bound to the metal surface. Adsorbed formaldehyde is released more easily than pars-formaldehyde from the wall of the autoclave by a decreasing gas phase concentration caused by treatment of an absorbing load in the autoclave whereby the adsorbed part of the formaldehyde is active as sterilant. Former reports dealing with formaldehyde gas phase measurements in the autoclave present a solution to the failing concentration during sterilization, which deviates from the one reported here. Weymes and coworkers4 used a dynamic process where a formalir+steam mixture was injected into the autoclave followed by a vacuum. These second-long pulses might create a higher average formaldehyde concentration than the usual static procedure presented earlier. Experiences from the present work show that a formaldehyde-steam mist is formed during fast pulsation as in the dynamic procedure. This fact prevents equilibrium gas phase measurement, being the aim of this study. Besides the drop in gas phase measurement Marcos and Wiseman’ reportformaldehyde layering which is not observed in this study. The observed layering is therefore probably the result of a concentration gradient caused by a cold spot in the autoclave.
Residues Previous values* for formaldehyde residues on sterilized goods have been based on extraction and depolymerization of monomeric formaldehyde and par&formaldehyde. This method has not permitted determination of the amount of formaldehyde dissolved in polymer materials such as PVC and PE, because of the small amounts and the slow release involved. The method described in the present paper makes this possible, and it also made it possible to distinguish between formaldehyde and pare-formaldehyde sorbed by the materials because of the initial depolymerization in sodium hydroxide solution. Measurements show that the adsorbed amount is less than of formaldehyde/par&formaldehyde 0.1 m cm -* after treatment in the autoclave, which is in agreement with previous findings’*‘. This is not surprising as the process is designed to remove the para-formaldehyde by steam pulsation. The solubility of formaldehyde in the plastic materials investigated decreases with falling polarity of
84
Biomaterials
1984,
Vol5
March
the polymerfrom 235 ppm in PVC to 8 ppm in HD-PE. The solubility of the polyethylene depends on the crystallinity of the material, being highest in LD-PE which has the lowest crystallinity of the polyethylenes investigated. The solubility is indicated for30 mg HCHO I-’ and due to the low values it is presumed to follow Henrys law which permits calculations for other concentrations. No variations were seen forthe solubilityof formaldehyde in the different PVC samples. This is in good agreement with expectations based on the minor changes of the solubility parameters for plasticized and semi-plasticized PVC”. On the other hand the diffusion coefficient varies with the plasticizer content, which was also observed for ethylene oxide in PVC. The diffusion coefficient measured is approx. 100 times smaller than the equivalent values for ethylene oxide in the same materials and is of the same order of magnitude as the diffusion coefficient of EO in poly(methyl methacrylate). The low diffusion coefficient for this system is known in practice to give aeration times of several years. The decimation time for ethylene oxide reduction in poly( methyl methacrylate) saturated with ethylene oxide is 3 years at 20°C for a 1 mm thick sheet. The calculation is based on data6 and an activation energy of 50 kJ mol-‘. Given the same activation energy for the diffusion of formaldehyde in PVC 30 a 90% saturation will take 250 h for a 1 mm thick sheet at 7O’C. and 10 h for an item of 0.2 mm thickness as used in the present investigation. Comparing the actual sterilization conditions, whereby goods seldom are exposed to formaldehyde for more than 30 min. at lower gas concentrations, with the conditions used for the present solubility measurements it is obvious that the amount of dissolved formaldehyde retrieved in sterilized polyolefines must be much less than 10 ppm and that PVC may contain 10 times higher concentrations. Accumulation at repeated sterilizations may occur due to the slow aeration and the relatively high sterilization temperature. The amounts of dissolved formaldehyde present will, however, be small compared to residues just after an EO-sterilization. The amounts of formaldehyde released to the users will likewise be small due to the value of the diffusion coefficient. It should be stressed that these considerations are valid for absorbed formaldehyde only, while surface adsorbed para-formaldehyde may cause release of formaldehyde to a much higher extent.
Airborne formaldehyde Measurements of airborne formaldehyde in front of formaldehyde autocalves show that some of the autoclaves described may exceed the occupational exposure limit (0.3 - 1 ppm in Denmark). However, it is an open question depending on the operation of the autoclave. One of the autoclaves examined is below the occupational exposure limit even at extremely high loads. The experimental autoclave showed concentrations about 1 ppm in unloaded condition, but when loaded with a large amount of cotton fabrics it rose to 6 ppm. The concentrations are also small when compared to measurements published for ethylene oxide around EOautoclaves. The occupational exposure around the examined autoclaves could be reduced essentially by
Formaldehyde
installation of a ventilation system, which was not used in any of the investigated facilities.
CONCLUSIONS The present investigation shows that it is possible to construct a formaldehyde autoclave which, just like an EOautoclave, can generate well-defined gas mixtures of sterilant and steam, and that this mixture remains stable with time at concentrations close to saturation. Measurement of gas residues is supplemented compared to previous measurements which concentrated on adsorbed para-formaldehyde. It has been possible to determine absorbed formaldehyde in concentrations down to 9 ppm in polyethylene which is the solubility at the conditions examined. The equivalent solubility for poly(vinyl chloride) is determined as 235 ppm, a value being sufficiently high to determine the diffusion coefficient for formaldehyde in this material. The quantity of airborne formaldehyde, which can be measured around some formaldehyde autoclaves under extreme conditions, is of the same order of magnitude as the occupational exposure limit. The actual exposure will thus be smaller than this value and can be reduced considerably by installation of a ventilation system.
sterilization:
K Handlos
REFERENCES 1
2
3
4
5
9
10
Deverill. C.E.A. and Cripps, N.F., Test on a low temperature steam and formaldehyde autoclave: the Mini clave 8O,/.Hosp./nfection 1981, 2, 175 Gibson, G.L. Processing heat-sensitive instruments and materials by low-temperature steam and formaldehyde, l.Hosp. Infection 1980, 1, 95 Handlos, V., Formaldehyde sterilization II. Formaldehyde-steam sterilization, the process and its influence on the formaldehyde residuals, Arch. Pharm.Chemi, Sci.ed. 1979, 7. 1 Weymes, C.. White, J.D. and Harris, C.. Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressures as a method of sterilising non porous heat sensitive items, Greater Glasgow Health Board Sterilkation Centre Note 4 (1975) Marcos, D. and Wiseman, D., Measurement of formaldehyde concentrations in a subatmospheric steam-formaldehyde autoclave, J. C/in. Pathol. 1979, 32, 567 Handlos, V., Kinetics of the aeration of ethylene oxide stenhzed plastics, Biomaterials 1980, 1, 149-l 57 NIOSH, Occupational exposure to formaldehyde, U.S. Department of Health, Education and Welfare, Washington 1976 Handlos, V., Formaldehyde sterilization I. Determinatron of forresiduals in maldehyde autoclave sterilrzed materials, Arch. Pharm. Chemi, Sci.ed. 1977, 5, 163 Handlos, V.. Formaldehyde sterilization III. The behaviour of the loaded autoclave and the permeability of plastic materials to formaldehyde, Arch. Pharm.Chemi, Sci.ed. 1979, 7. 12 Schott, H., Solubility parameter, specific molar cohension and the solubility of ethylene oxide in polymers, Biomaterials 1982, 3, 195-l 98
Eiomaterials
1984.
Vol 5 March
85
The design of a sterilizer for sterilization
Universitetsparken
2, DK-2 100 Copenhagen,
of heat sensitive items using gaseous formaldehyde
cribed. The sterilizer is able to create a constant formaldehyde-steam hour. It is further able to operate at formaldehyde
concentrations
concentration
close to saturation with only small residues on of formaldehyde
poly(vinyl chloride) being approx 15 and 250 ppm, respectively,
at 70°C and 30 mg WHO
of formaldehyde
and steam is des-
over a period of at least one
sterilized plastic materials. The autoclave is used for measurement coefficient
Denmark
solubility in polyolefines and f’.
The diffusion
chloride) is measured and is in the same order of magnitude as for ethylene oxide in poly(methyl methacrylate) ie. 10-l” cd 2’. M easurements of airborne formaldehyde in front of different sterilizers
Keywords:
in poly(vinyl
show that it is possible to design facilities
Sterilization,
formaldehyde,
diffusion
1984
Euttemvorth
Et Co (Publishers)
exposure limits.
coefficient
Due to drawbacks arising from the usage of ethylene oxide as a sterilant a growing interest has been registered for the usage of formaldehyde as a substitute”2. Formaldehyde at high relative humidity has been used for sterilization of heat sensitive items in British and Scandinavian hospitals for many years. The use of the method has been limited, by the uncertainty as to its microbiological efficiency which has been tested only to a limited extent under circumstances which can be reproduced. There is still uncertainty on the toxicological aspect of the method arising from gas residual on sterilized devices and exposure of the personnel to airborne formaldehyde. The lack of reproducibility is due to the fact that gaseous formaldehyde, in mixtures with steam or alone, is a complex physicaCchemical system due to its very high water solubility and its ability to polymerize3. This is the reason why the gas phase concentrations in the autoclaves, calculated on the basis of the amount of formalin added, are very seldom in agreement with the actual measured gas phase concentrations. Published results3-5 on measurements of gaseous formaldehyde in sterilizers show that the concentration of formaldehyde even in the unloaded autoclave decreases in the course of time. This indicates that formaldehyde evaporated in the autoclave is removed either by polymerization which takes place on the cold parts of the autoclave or by dissolution of formaldehyde in steam condensates. Measurements of formaldehyde gas phase concentrations in an autoclave during sterilization are thus a useful method for control of both the physical and the chemical conditions in the autoclave. The toxicological evaluation of formaldehyde as a sterilant is divided in two parts, one dealing with the 0
meeting occupational
exposure to airborne formaldehyde of the personnel and the other dealing with formaldehyde residues in the sterilized goods. Occupational exposure limits vary from country to country, but are approximately 1 ppm or identical with the olfactory limit. No official limits for formaldehyde residues in sterilized goods have been published. The purpose of the present investigation has been to design an autoclave which unloaded can keep a constant formaldehyde gas phase concentration during sterilization, to supplement previous measurements of residual formaldehyde in thermoplastics and to measure airborne formaldehyde.
EXPERIMENTAL Apparatus
PROCEDURE
and chemicals
The autoclave was made by Getingeverken, Sweden, for experimental use only. Chamber and door, fully jacketed, were made of stainless steel, whereas pipes and fittings were made of copper and brass. The cylindrical chamber (100 I) is fitted with a vaulted bottom in one end and a door in the other. The longitudinal axis of the chamber is tilted against the horizontal plane to facilitate the catching of condensate in the bottom valve, which is built into the heated jacket to prevent cold spots. The steam and formaldehyde leading pipes are equipped either with a steam-heated jacket (1 10°C) or are built into the water jacket of the chamber. Steam generator and water jacket are both electrically heated. The functions of the autoclave (cycles, pressure, temperature, etc.) are programmed to operate automatically. Pressure and temperature
Ltd. 0142-9612/84/050081-05$03.00 Biomaterials
1984,
Vol 5 March
81
Formaldehyde
sterilization:
K Handlos
3 mmHg and & 0.5”C) in the autoclave are registered continuously. The gas chromatograph was a PYE model 44 provided with a hot wire detector and operated under the following conditions: columns bore 4 mm, length 1.5 m, packed with Porapack T; carrier gas nitrogen, flow rate 35 ml min.-‘; column temperature 120°C; detector temperature 150°C; detector current 11 OmA. The autoclave was connected to the gas chromatograph by means of a l/l 6” stainless steel pipe transmitting the gas direct from the autoclave chamber to a gas sampling valve, from where a sample of 1.81 ml was injected onto the column. The pipe connecting the autoclave and the gas chromatograph had a 0.5 m free flexible end for sampling in the chamber. Throughout its length the pipe was heated to minimum 110°C by an 8 A current. The sampling loop and the gas valve were immersed in a silicone oil thermostat of 130+l”c. The sample oxidizer was a Packard Model 3306 Tricarb and the scintillation counter a Beckmann LS 200. The formaldehyde was used as a non-methanol stabilized formalin containing 37.4% HCHO; the “C-labelled formaldehyde as a 1.3% aqueous solution, specific activity 1 .13 mCi/M HCHO from Amersham Int. The scintillation liquids were Pica-fluor 15 from Packard and Lipoluma from Lumac. The composition of the plastic materials is given in the references6. (k
Methods The autoclave is operated at a jacket temperature of 70°C and all cycles start with three steam pulsations between 50 and 245 mmHg for air removal. During the subsequent vacuum the formalin is injected into the steam heated U-pipe. The autoclave is then filled with steam to the pressure of saturated steam plus the partial pressure of the added formaldehyde calculated as an ideal gas. The sterilization phase is followed by a cleaning of the autoclave by 6 steam pulsations varying between lOO245 mmHg. The gas chromatographic determination of formaldehyde and water vapour in the autoclave atmosphere is made using helium as internal standard, and 250 ml of helium (2O’C. 760 mmHg) is added with the formalin. Standard deviation of the gas chromatographic determination of formaldehyde is 2% at a concentration of 30 mg HCHO I-‘. Calibration of the gas chromatograph was carried out with known amounts of formaldehyde in aqueous solution, detector response is determined as peak areas. Determination of the formaldehyde concentration in a hospital autoclave was made by attachment of a pipe furnished with a valve at each end to the top of the autoclave. The pipe was heated to above 80°C (jacket temperature of the autoclave) and the free end connected to a vacuum pump drawing a small amount of the autoclave atmosphere through the pipe. By closing the valves a welldefined amount of autoclave atmosphere was trapped and made available for analysis. Determination of airborne formaldehyde in front of the autoclaves is carried out by the impinger method as described by NIOSH’. The sampling period was 20 min. starting just after termination of the sterilization cycle and opening of the autoclave door. Samples were collected just in front of the door. Saturation of the polymer samples with partially
82
8iomaterial.s
1984,
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“C-labelled formaldehyde was made by addition of 20 $i labelled HCHO to IO ml of formalin. After injection into the autoclave this mixture gave a gas phase concentration of 30 mg HCHO I-‘. In this steam-formaldehyde atmosphere polyolefine and poly(vinyl chloride) samples were saturated with formaldehyde during a 6 h exposure. After the treatment the PVC samples were dissolved in dimethyl formamide (0.7 g in 10 ml) and 1.5 ml of this solution and 2 ml of scintillation liquid were counted for 14C decay. The polyolefine samples (70 mg) were burned in the Packard sample oxidizer and the amount of 14Clabelled carbon dioxide was counted after scintillation liquid addition. All results were adjusted for counting efficiency and retrieval adopted by standards.
RESULTS Measurement of formaldehyde gas phase concentrations in an unloaded hospital autoclave daily operating according to Scandinavian sterilization standards shows a quick decrease in formaldehyde gas phase concentration as a function of time after addition of formalin (Figure 7). The autoclave runs at 80°C and formalin is added corresponding to 60 mg HCHO I’-’ autoclave volume. Figure 7 shows the variations between two identical empty runs and that after 25 min. the formaldehyde concentration is less than 1% of the expected value. This finding is in agreement with previously published results3. Despite the low formaldehyde concentration in the gas phase the autoclave is capable of sterilizing goods on a routine basis. Measurements of the temperature in the chamber of the experimental autoclave show that all parts of the chamber have a temperature within 0.5”C of the set temperature. All pipes connected to the chamber are heated to the temperature of the chamber orto 110°C except for a 30 mm %” pipe which is heated to only 40°C. Pressure measurements in the chamber show a leakage corresponding to maximum 4 mmHg h-’ at 180 mmHg. The measurement of formaldehyde gas phase concentration after evaporation of 10 ml of formalin in the autoclave shows (Table 7) that the gas phase concentration is constant during the first hour. Moreover, the measurements show no concentration gradient between bottom and top of the autoclave. The measurements were carried out in three different runs, the sampling tube being placed at different positions as the tube cannot be moved once the autoclave is closed. The results for independent
15
I
Figure 1 autoclave
Formaldehyde
gas
phase
concentration
in
a hospital
Formaldehyde
Table 1
Gas phase
concentration
of formaldehyde
as a function
of
Gas phase concentration top
0 10 20 30 40 50 60
<
0.1 30.0 30.2 30.7 30.0 30.0 31.3
Solubility
of formaldehyde
in plastics
V: Handlos
at 70°C
and 30 mg
HCHO r’
time after addition of 10 ml of formalin Time/min.
Table 2
steriltzation:
mg/l
centre
bottom
<
<
0.1 31 .o 30.0 30.7 31.7 31.7 30.0
0.1 30.7 30.7 30.7 29.8 30.0 30.7
runs are reproducible, but show a gas phase concentration which is 25% smaller than expected according to the amount of formalin added (40 mg HCHO I-‘). The results in Table 7 are not affected by an air intake equivalent to 100 mmHg. The gas phase concentration of formaldehyde after addition of formalin in portions of 2 ml until saturation with steam is illustrated in Figure 2, where these values are compared with calculated gas phase concentrations based on the added amount After addition of 18 ml of formalin the autoclave is saturated with steam; the water vapour pressure in the autoclave was approx. 90 mmHg before the formalin intake. Addition of more formalin gives varying gas phase results due to mist formation in the autoclave. The constant formaldehyde concentration in the gas phase was utilized to expose 0.2-0.4 mm thick plastic films [polyethylene (PE), polypropylene (PP) and poly(vinyl chloride) (PVC) with content of plasticizer from 30-60 parts per 100 parts of polymer] to “C-labelled formaldehyde. After treatment for 6 h the samples were saturated with formaldehyde as longer treatment did not increase the uptake. After the 6 h period the gas phase concentration of formaldehyde was 90% of the initial value. After removal from the autoclave the samples were washed with 0.2 M sodium hydroxide before the formaldehyde assay to remove anypara-formaldehyde from the surface. Noformaldehyde was detected in the alkaline water extract (< 0.1 pg cm-‘). Table 2 shows the formaldehyde content of the saturated samples as well as the counts in counts per minute (CPM); the background for the oxidized samples was 16 CPM and 11 CPM for dissolved samples. Standard deviations for counts on polyolefines were 15% and for the PVC samples 3%.
LD- PE MD-P-t HD-PE PP PVC
16 12 8 24 235
(PgLglg)
CPM 18 12 9 27 279
DISCUSSION Gas phase measurements The gas phase measurements in Table 1 and Figure 2 show that it is possible to construct an autoclave for practical purposes with an almost saturated gas phase concentration being reproducible and constant over a period of time essentially longer than the normal sterilization period (30 min.). This has previously not been the case, as the literatures5 and the examined hospital autoclave indicate. This result is achieved by heating the surface of the autoclave chamber, pipes and valves in contact with steam and formaldehyde. One single pipe was not heated, which did not seem to influence the result This stresses the possibility of constructing equipment for practical use. In spite of this progress it is still not possible to get a 100% retrieval of the added formaldehyde in the chamber atmosphere. The retrieval is approx. 75% at 30 mg HCHO I-‘. It has previously been stated3 that the reason for the low retrieval of formaldehyde in the gas phase was due to dissolution
of monomeric In the actual
concentration mechanisms centration
formaldehyde
and polymerization
autoclave. in the experimental of 2 ml. Solid line:
Solubility
The formaldehyde concentration in plasticized PVC was measured at 27 and 146 h after saturation using the radio chemical method, the samples were kept at 22°C in formaldehyde free surroundings. The value after 27 h was 24 ppm and after 146 h 9 ppm for 0.2 mm thick PVC 30 (30 g plasticizer per 100 g polymer). The corresponding values for PVC 60 were 42 and 12 ppm for a 0.34 mm thick film. If the diffusion coefficient is calculated on the basis of a straight line aeration course in a semilogarithmic plot of concentration versus time, the resulting coefficients are 1 X IO-” cm2 s-’ for PVC 30 and 3 X 1 O--” cm2 s-’ for PVC 60. Measurement of airborne formaldehyde in front of the unloaded experimental autoclave showed variations between 0.7 and 1.2 ppm (~1 HCHO I-’ air). Loaded with e.g. a lab coat (610 g cotton) the variations were between 2.5 and 6.2 ppm and loaded with 1 kg of disposable polyolefine syringes the concentrations varied between 1.8 and 2.7 ppm. Four hospital autoclaves, ranging in size from 20 I to 5 m3 fell into two groups according to their formaldehyde release. Three of them showed concentrations from 2.5 to 3.5 ppm both in loaded and unloaded conditions. One autoclave, volume 300 I, varied between 0.03 ppm in unloaded condition and 0.9 ppm when loaded with four lab coats.
densate
Figure 2 Formaldehyde gas phase concentration autoclave after addition of formalin in quantities calculated values
Material
rules would
case
out cause
as a function
in the steam
in the non-hebted the constant
these
of time.
Riomaterials
formaldehyde
explanations,
a decreasing
gas
An inadequate
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parts of the as
both
phase
con-
release of
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Formaldehyde
sterilization:
K Handlos
formaldehyde from the formalin added would give an increasing formaldehyde concentration as a function of time, as the non-evaporated formalin would be in contact with surfaces above 70°C. At this temperature the concentration of saturated formaldehyde vapour overformalin orpara-formaldehyde is from 50-60 mg HCHO I-’ (Refere”ce 3). Thus the autoclave is unsaturated with formaldehyde in the present experiment. It is most likely that the disagreement between measured and calculated gas phase concentration is due to adsorption of monomeric formaldehyde to the metal parts of the autoclave. Figure 2 shows that the retrieved part of formaldehyde falls from approx. 95% at 8 mg HCHO I-’ to 75% at 30 mg HCHO I-‘. This indicates that formaldehyde is adsorbed onto the wall of the autoclave in competition with steam as the percentage content of formaldehyde in the gas rises with increasing amount of formalin. The gas initially is pure steam. The explanation of adsorption implies that by 30 mg of HCHO I-’ approx., 80 w HCHO cm-* is bound to the metal surface. Adsorbed formaldehyde is released more easily than pars-formaldehyde from the wall of the autoclave by a decreasing gas phase concentration caused by treatment of an absorbing load in the autoclave whereby the adsorbed part of the formaldehyde is active as sterilant. Former reports dealing with formaldehyde gas phase measurements in the autoclave present a solution to the failing concentration during sterilization, which deviates from the one reported here. Weymes and coworkers4 used a dynamic process where a formalir+steam mixture was injected into the autoclave followed by a vacuum. These second-long pulses might create a higher average formaldehyde concentration than the usual static procedure presented earlier. Experiences from the present work show that a formaldehyde-steam mist is formed during fast pulsation as in the dynamic procedure. This fact prevents equilibrium gas phase measurement, being the aim of this study. Besides the drop in gas phase measurement Marcos and Wiseman’ reportformaldehyde layering which is not observed in this study. The observed layering is therefore probably the result of a concentration gradient caused by a cold spot in the autoclave.
Residues Previous values* for formaldehyde residues on sterilized goods have been based on extraction and depolymerization of monomeric formaldehyde and par&formaldehyde. This method has not permitted determination of the amount of formaldehyde dissolved in polymer materials such as PVC and PE, because of the small amounts and the slow release involved. The method described in the present paper makes this possible, and it also made it possible to distinguish between formaldehyde and pare-formaldehyde sorbed by the materials because of the initial depolymerization in sodium hydroxide solution. Measurements show that the adsorbed amount is less than of formaldehyde/par&formaldehyde 0.1 m cm -* after treatment in the autoclave, which is in agreement with previous findings’*‘. This is not surprising as the process is designed to remove the para-formaldehyde by steam pulsation. The solubility of formaldehyde in the plastic materials investigated decreases with falling polarity of
84
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the polymerfrom 235 ppm in PVC to 8 ppm in HD-PE. The solubility of the polyethylene depends on the crystallinity of the material, being highest in LD-PE which has the lowest crystallinity of the polyethylenes investigated. The solubility is indicated for30 mg HCHO I-’ and due to the low values it is presumed to follow Henrys law which permits calculations for other concentrations. No variations were seen forthe solubilityof formaldehyde in the different PVC samples. This is in good agreement with expectations based on the minor changes of the solubility parameters for plasticized and semi-plasticized PVC”. On the other hand the diffusion coefficient varies with the plasticizer content, which was also observed for ethylene oxide in PVC. The diffusion coefficient measured is approx. 100 times smaller than the equivalent values for ethylene oxide in the same materials and is of the same order of magnitude as the diffusion coefficient of EO in poly(methyl methacrylate). The low diffusion coefficient for this system is known in practice to give aeration times of several years. The decimation time for ethylene oxide reduction in poly( methyl methacrylate) saturated with ethylene oxide is 3 years at 20°C for a 1 mm thick sheet. The calculation is based on data6 and an activation energy of 50 kJ mol-‘. Given the same activation energy for the diffusion of formaldehyde in PVC 30 a 90% saturation will take 250 h for a 1 mm thick sheet at 7O’C. and 10 h for an item of 0.2 mm thickness as used in the present investigation. Comparing the actual sterilization conditions, whereby goods seldom are exposed to formaldehyde for more than 30 min. at lower gas concentrations, with the conditions used for the present solubility measurements it is obvious that the amount of dissolved formaldehyde retrieved in sterilized polyolefines must be much less than 10 ppm and that PVC may contain 10 times higher concentrations. Accumulation at repeated sterilizations may occur due to the slow aeration and the relatively high sterilization temperature. The amounts of dissolved formaldehyde present will, however, be small compared to residues just after an EO-sterilization. The amounts of formaldehyde released to the users will likewise be small due to the value of the diffusion coefficient. It should be stressed that these considerations are valid for absorbed formaldehyde only, while surface adsorbed para-formaldehyde may cause release of formaldehyde to a much higher extent.
Airborne formaldehyde Measurements of airborne formaldehyde in front of formaldehyde autocalves show that some of the autoclaves described may exceed the occupational exposure limit (0.3 - 1 ppm in Denmark). However, it is an open question depending on the operation of the autoclave. One of the autoclaves examined is below the occupational exposure limit even at extremely high loads. The experimental autoclave showed concentrations about 1 ppm in unloaded condition, but when loaded with a large amount of cotton fabrics it rose to 6 ppm. The concentrations are also small when compared to measurements published for ethylene oxide around EOautoclaves. The occupational exposure around the examined autoclaves could be reduced essentially by
Formaldehyde
installation of a ventilation system, which was not used in any of the investigated facilities.
CONCLUSIONS The present investigation shows that it is possible to construct a formaldehyde autoclave which, just like an EOautoclave, can generate well-defined gas mixtures of sterilant and steam, and that this mixture remains stable with time at concentrations close to saturation. Measurement of gas residues is supplemented compared to previous measurements which concentrated on adsorbed para-formaldehyde. It has been possible to determine absorbed formaldehyde in concentrations down to 9 ppm in polyethylene which is the solubility at the conditions examined. The equivalent solubility for poly(vinyl chloride) is determined as 235 ppm, a value being sufficiently high to determine the diffusion coefficient for formaldehyde in this material. The quantity of airborne formaldehyde, which can be measured around some formaldehyde autoclaves under extreme conditions, is of the same order of magnitude as the occupational exposure limit. The actual exposure will thus be smaller than this value and can be reduced considerably by installation of a ventilation system.
sterilization:
K Handlos
REFERENCES 1
2
3
4
5
9
10
Deverill. C.E.A. and Cripps, N.F., Test on a low temperature steam and formaldehyde autoclave: the Mini clave 8O,/.Hosp./nfection 1981, 2, 175 Gibson, G.L. Processing heat-sensitive instruments and materials by low-temperature steam and formaldehyde, l.Hosp. Infection 1980, 1, 95 Handlos, V., Formaldehyde sterilization II. Formaldehyde-steam sterilization, the process and its influence on the formaldehyde residuals, Arch. Pharm.Chemi, Sci.ed. 1979, 7. 1 Weymes, C.. White, J.D. and Harris, C.. Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressures as a method of sterilising non porous heat sensitive items, Greater Glasgow Health Board Sterilkation Centre Note 4 (1975) Marcos, D. and Wiseman, D., Measurement of formaldehyde concentrations in a subatmospheric steam-formaldehyde autoclave, J. C/in. Pathol. 1979, 32, 567 Handlos, V., Kinetics of the aeration of ethylene oxide stenhzed plastics, Biomaterials 1980, 1, 149-l 57 NIOSH, Occupational exposure to formaldehyde, U.S. Department of Health, Education and Welfare, Washington 1976 Handlos, V., Formaldehyde sterilization I. Determinatron of forresiduals in maldehyde autoclave sterilrzed materials, Arch. Pharm. Chemi, Sci.ed. 1977, 5, 163 Handlos, V.. Formaldehyde sterilization III. The behaviour of the loaded autoclave and the permeability of plastic materials to formaldehyde, Arch. Pharm.Chemi, Sci.ed. 1979, 7. 12 Schott, H., Solubility parameter, specific molar cohension and the solubility of ethylene oxide in polymers, Biomaterials 1982, 3, 195-l 98
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