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Taking the hazard out of Cl andClO2 exposures in a kraft pulp mill

Increased respiratory symptoms and airflow obstruction, particularly among non-smokers and former smokers can be caused by accidental chlorine or chlorine dioxide (Cl or ClO2) exposure.To validate est...

March 1, 1999
By Pulp & Paper Canada


Increased respiratory symptoms and airflow obstruction, particularly among non-smokers and former smokers can be caused by accidental chlorine or chlorine dioxide (Cl or ClO2) exposure.

To validate estimates of exposure contained in a Job Exposure Matrix for the pulp and paper industry in British Columbia, 1678 randomized personal measurements were made for five common pulp mill contaminants: hydrogen sulphide, carbon monoxide, calcium oxide, wood dust, and chlorine/chlorine dioxide. The samples were collected during 73-day shifts for 46 randomly selected job titles at a bleached kraft pulp mill. Midget impingers containing a buffered solution of potassium iodide[1] were used as area samplers to quantify the shift-long (12 hour) time weighted average (TWA) for ClO2 and Cl2. Data-logging electro-chemical sensors (ECS) and passive-detector cards (PDC) were used as personal breathing-zone samplers. The ECS are capable of quantifying exposure in three simultaneous modes: shift-long TWA, 15 minute TWA (TWA15min), and “peak” exposures. PDC quantify the shift-long TWA. Details of the study have been published.[2,3]

Personal exposures to the above mentioned substances were generally less than half the shift-long TWA Exposure Limits[3] (EL) established by the Workers’ Compensation Board of British Columbia (WCB)[4].

Even in modern automated processes, there are occasional accidental spills of stock and leaks in process equipment leading to transient and sometimes high concentrations of hazardous gases. Worker exposures during such incidents are referred to in the industry as “gassings.” In the past, measurements were commonly made using detector tubes. With the advent of data-logging personal samplers, transient personal exposures during normal production duties and gassings can be measured in a continuous manner during the course of a shift.

Using such instrumentation, excessive TWA15min and peak personal exposures to ClO2 were documented in a study during day-to-day tasks in the chemical preparation area. High Cl2 exposures were also documented in the bleach plant.

Hazards of Cl2 and ClO2

ClO2 and Cl2 are powerful oxidizers. Both of these agents are documented in the occupational hygiene literature as irritants of the respiratory system and of the eyes, nose and throat[5]. In BC, the WCB exposure limit for ClO2 for 12 hour shifts (EL12) is 0.05 parts per million (ppm). The EL for a TWA15min is 0.3 ppm.[4] The level that is of immediate danger to life and health (IDLH) is 5 ppm.[6] For Cl2, the EL12 is 0.25 ppm and the EL for a TWA15min is 1.0 ppm. The IDLH is 10 ppm.[5] Good hygiene practice dictates that at no time should workers be exposed to greater than five times the eight-hour TWA, 0.1 ppm for ClO2, 0.5 ppm for Cl2.[7]

The literature distinguishes the effects of one-time acute exposures to ClO2 and Cl2 and chronic exposures. Schwartz et al[8] reported on 20 previously healthy construction workers who were accidentally exposed to chlorine gas in a pulp mill during unloading of a tank car. Pulmonary function tests conducted one day after, and up to 12 years post-exposure, suggested that acute exposure to high concentrations of Cl2 may result in long-term pulmonary complications characterized by reduced residual volume (RV). In a follow-up to this study, Charan et al[9] note that in some of the patients, acute exposure to Cl2 caused immediate changes in lung functions, which gradually resolved due to the healing process.

A number of authors have commented on the effects of repeated gassings of ClO2 and Cl2 in pulp mills. Kennedy et al[10] collected personal breathing-zone samples using long-term detector tubes over a month-long period and showed TWA Cl2/ClO2 levels to be less than 1.0 ppm. However, 60% of the pulp mill workers reported one or more Cl2/ClO2 gassing incidents and these workers were significantly more likely to report wheezing than control groups. The authors concluded that accidental Cl2/ClO2 gassings are associated with increased respiratory symptoms and airflow obstruction, particularly among non-smokers and former smokers. Salisbury et al[11], studied the same population using pulp mill first aid records spanning an eight-year period and found a significant decrease in forced expiratory volume in one second (FEV1)/forced vital capacity (FVC) among workers who had at least one chlorine or chlorine dioxide gassing incident.

Henneberger et al[12] reported similar results, with accidental gassings to Cl2, ClO2 and other pulp mill toxic gases being associated with decreased FEV1 and FVC, independent of smoking. Chan-Yeung et al[13] followed three pulp mill workers, each from a different mill, who had multiple gassings over a period of years. Each of the workers had at least one episode for which emergency room treatment was required. The results of this study indicated that asthmatic symptoms, variable airflow obstruction, and non-specific bronchial hyper-responsiveness can arise from a series of multiple exposures to high levels of pulp mill irritant gases.

Courteau et al[14] reported that 71 construction workers repeatedly exposed to chlorinated gases in the bleach plant over a three to six month period were at moderate to high risk of developing chronic lung disease; laboured breathing (dyspnoea) developed among exposed workers independent of smoking, age, and asthmatic status. In a follow-up study of the same population, Bhrer et al[15] found that 18 to 24 months after exposure ended 82% still had respiratory symptoms, 23% had evidence of bronchial obstruction, and 41% had bronchial hyper-responsiveness.

Generation of ClO2

ClO2 is the exclusive bleaching agent in the mill where the sampling program was carried out. Since ClO2 is unstable and explosive, it is produced on-site. In the “R8” process, sodium chlorate, methanol, sulphuric acid and a small amount of sodium chloride as a catalyst are metered into the generator. A series of three different reactions occur. In the principal and desired reaction, sodium chlorate is completely converted to ClO2 as described by:

3 NaClO3 + 2 H2SO4 + 0.85 CH3OH 3ClO2 + Na3H(SO4)2 + 2.2 H2O + 0.06 CH3OH + 0.52 HCOOH + 0.27 CO2.

However, the other two reactions produce an undesirable small amount of Cl2. Under controlled reaction conditions, this Cl2 content can be minimized. In addition, small amounts of Cl2 can enter the mill environment through two other means: the decomposition of fugitive ClO2 by ultraviolet light[16,17,18] or by contact with organic fines such as wood and cellulose dust in mill air.

At the time of the study, no personal air sampling technology was available to measure ClO2 exclusively. Sampling devices designed to measure Cl2 but cross-sensitive to ClO2 were chosen to measure bleach chemical exposures. The instruments selected were PDC (the Envirometrics ACT Monitoring Card System) and data-logging ECS (the Biosystems Toxilog). The ECS were programmed to log one data point every 30 seconds.

Response ratios between Cl2 and ClO2 for these instruments were ascertained by the National Council for Air and Stream Improvement (NCASI). The ECS with a Cl2 sensor responded to ClO2 at a ratio of 2:1. The PDC responded to ClO2 at a ratio of 1.8:1[19,20].

To assure that air samples collected were in fact ClO2, a method capable of quantifying both ClO2 and Cl2 was used. This method used midget impingers filled with a solution of potassium iodide and analyzed by ion chromatography. The bubblers were made of glass containing a liquid reagent and hence were not suitable as personal samplers.

In the chemical preparation area, 70% of the ClO2 ECS personal measurements detected at least one TWA15min exposure greater than the EL. Figures 1A and 1B show typical ClO2 profiles for different chemical preparation operators on different days. In both instances, multiple TWA15min exposures greater than the EL were recorded. The highest peak value in Fig. 1B was 13.6 ppm ClO2. None of the ECS measurements showed TWA12 personal exposures to be greater than the EL. For PDC, 14% of the personal samples showed a TWA conce
ntration greater than the EL12 and 25% of the samples were greater than half (action level) of the EL12. In the bleach plant, 12 ECS measurements detected peak exposures greater than the Cl2 TWA15min.

A major pulp spill occurred during the 73-day sampling period. An ECS carried by a clean-up operator registered multiple consecutive TWA15min exceeding the ClO2 EL. The highest peak exposure was 2.4 ppm. The shift-long average, however, was below the detection limit (<0.2 ppm).

Six impingers were also distributed as TWA area samples throughout the bleach plant basement during the spill clean-up. Analysis of these samples showed both ClO2 and Cl2 were present. The highest concentrations of ClO2/Cl2 were 0.22/0.09 ppm, with a mean concentration of 0.09/0.03 ppm. The highest impinger measurements of ClO2 were closest to the spill clean-up area. Conversely, impinger measurements of Cl2 were highest further away from the immediate clean-up area, indicating a gradient of conversion of ClO2 into Cl2.


As noted above, worker TWA15min and peak over-exposures to inorganic chlorinated gases were identified in two separate scenarios: during regular production rounds in the chemical preparation and bleach plant areas and during process upset conditions in the basement of the bleach plant. Prior to the availability of ECS, such measurements were not feasible due to the limitations of industrial hygiene sampling equipment.

Exposures during daily tasks: In the chemical preparation area, workers spend the greater part of their 12 hour shift in positively-pressured control rooms in which room air is drawn from fresh-air sources. Every two hours an operator leaves the control room to conduct rounds. Part of the duty is to collect a process sample of ClO2 from the generator and to analyse it in the lab for bleach strength. During the sampling program, the TWA for the 12 hour shift was consistently below the EL12, yet each time operators collected process samples, the TWA15min exceeded the EL and peaks were frequently greater than five times the TWA. Figures 1A & 1B demonstrate these peak and TWA15min exposures. These figures also demonstrate that different operators experienced the same patterns of exposure but different exposure concentrations, presumably because of individual technique in drawing the sample.

While conducting subsequent compliance sampling in two other bleached kraft pulp mills, one of the authors (LMS) noted a similar pattern of TWA15min and peak over-exposure to ClO2 occurring during bleach line sample collection.

Exposure during a pulp spill: During the study, the major pulp spill (20t) was caused by an expansion joint at the bottom of a bleaching tower rupturing. Portions of the bleach plant basement were waist-deep in pulp containing characteristically-coloured streaks of bleach. After repair of the joint, clean-up operations began by washing the pulp into mill sewers with hoses. Since area alarms were sounding, clean-up personnel wore self-contained breathing apparatus (SCBA). When area alarms stopped, the bleach plant basement was declared “safe” and respiratory protection was removed. Study investigators then distributed personal sampling devices to workers; six impinger area samplers were also placed throughout the area where the spill occurred.

Impinger samplers detected the presence of both ClO2 and Cl2. Concentrations of these gases varied from sampler to sampler, indicating non-homogeneous air concentrations of inorganic chlorinated gases throughout the area. It is presumed that after elevated amounts of inorganic chlorinated gases vaporized from the spilled pulp, area alarms located away from the immediate clean-up area no longer detected high enough concentrations to keep sounding. However, as workers hosed down the pulp, localized pockets of bleach chemical were liberated, resulting in worker exposure.

Thus, area continuous-monitors can serve a valid alarm function when warning is required to prevent entry into danger areas. However, the danger of transient high concentrations of hazardous gases is still present during the clean-up of pulp spills. Continuous-monitoring ECS equipped with alarms can be used as part of the decision-making process to determine the types of respiratory protection required to protect workers.


In summary, we identified and quantified several sources of excessive worker exposure to ClO2 and Cl2 in the chemical preparation and bleach plant areas of a pulp mill due to gassings in day-to-day production work and during a process upset. Continuous personal monitoring by data-logging electro-chemical sensors permitted this data to be recorded. These high levels have not been previously observed due to the limitations of older-generation industrial hygiene sampling equipment. In light of the above findings and the current knowledge of the health effect of gassings upon the respiratory systems, we recommend that:

1. Hygiene sampling programs be conducted to detect not only shift-long time-weighted average exposures but also to identify short-term and peak exposures workers can encounter during their normal tasks.

2. Local exhaust ventilation for sampling stations should be checked regularly to ensure there is adequate flow to prevent hazardous gas over-exposures. Local exhaust ventilation should be installed on sampling stations where none exists.

3. Personal monitoring instrumentation, such as ECS equipped with alarms, should be used to determine the need for respiratory protection during the clean-up of large pulp spills.

4. Education should be increased among pulp workers to inform them of the adverse health effects of multiple episodic exposures of short duration but high exposures to chlorinated and other hazardous gases.

5. Prospective spirometry programs should be started, including in the employment physical to identify loss of respiratory function among workers exposed to hazardous gases.

6. Further research is needed to determine how respiratory function is affected by multiple gassings from pulp mill hazardous gases.

This project was funded in part by the National Health and Research Development Program, Health Canada (grant 6610-2063-502) and the WCB of British Columbia. It was supported by the Forest Industrial Relations Health Program, the Pulp and Paper Workers of Canada, and mill management. Additional support came from the Laboratory Services of the Workers’ Compensation Board of BC, NCASI, and Dr. Robert Fisher.P&PC


1. Workers’ Compensation Board of British Columbia, Laboratory Analytical Methods, Laboratory Services. 1989.

2. Astrakianakis. G.; Svirchev, L.; Tang, C.; Janssen, R.; Anderson, J.T.L.; Band, P.; Le, N.; Fang, R.; Bert, J. Industrial Hygiene Aspects of a Sampling Survey at a Bleached-Kraft Pulp Mill in British Columbia. Accepted in American Industrial Hygiene Journal, 1998. In Press, Oct. 1998.

3. Astrakianakis, G.; Band, P.R.; Le, N.; Bert, J.; Janssen, R.; Svirchev, L.; Tang, C.; Anderson, J.T.L.; Keefe, A.R.: Validation of a Mill-Specific Job-Exposure Matrix in the British Columbia Pulp and Paper Industry. Appl. Occup. Environ. Hyg. 13(9):671-677; 1998.

4. Occupational Health and Safety Regulation, BC Regulation 296/97. Part 5, Section 549 (b). Workers’ Compensation Board of British Columbia, Vancouver, B.C. 1997.

5. Documentation of the Threshold Limit Values and Biological Exposure Indices. 6th Edition, ACGIH 1991.

6. Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs). Ludwig, H.R.; Cairelli, S.G.; Whalen, J.J. National Institute for Occupational Safety and Health (NIOSH) Cincinnati, May 1994.

7. 1998 TLVs and BEIs, Threshold Limit Values for Chemical Substances and Physical Agents. American Conference of Governmental Industrial Hygienists, Cincinnati, 1998.

8. Schwartz, D.A.; Smith, D.D.; Lakshminarayan, S. The Pulmonary Sequelae Associated with Accidental Inhalation of Chlorine Gas. Chest 1990; 97:820-25.

9. Charan, N.B.; Lakshmininaryan, S.; Myers, G.C.; Smith, D.D. Effects of Accidental Chlorine Inhalation on Pulmonary Function. West. J. Med
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10. Kennedy, S.M.; Enarson, D.A.; Janssen, R.G.; Chan-Yeung, M. Lung Consequences of Reported Accidental Chlorine Gas Exposures among Pulpmill Workers. Am. Rev. Respir. Dis.143:74-79; 1991.

11. Salisbury, D.A.; Enarson, D.A.; Chan-Yeung, M.; Kennedy, S.M. First Aid Reports of Acute Chlorine Gassing among Pulpmill Workers as Predictors of Lung Health Consequences. Am. J. I. Med. 20: 71-81, 1991.

12. Henneberger, P.K.; Ferris, B.G.; Sheehe, P.R. Accidental Gassings Incidents and the Pulmonary Function of Pulpmill Workers. Am. Rev. Respir. Dis. 148:63-67; 1993.

13. Chan-Yeung M, Lam S, Kennedy SM, Frew AJ. Persistent Asthma after Repeated Exposure to High Concentrations of Gases in Pulpmills. Am J Crit Care Med. Vol 149. pp 1676-1680, 1994.

14. Courteau, J-P.; Cushman, R.; Bouchard, F.; Quvillon, M.; Chartrand, A.; Bhrer, L.; Survey of Construction Workers Repeatedly Exposed to Chlorine over a Three to Six Month Period in a Pulpmill: I. Exposure and Symptomatology. Occup. Environ. Med.; 51:219-224; 1994.

15. Bhrer, L.; Cushman, R.; Courteau, J-P.; Quvillon, M.; Ct, G.; et al. Survey of Construction Workers Repeatedly Exposed to Chlorine over a Three to Six Month Period in a Pulpmill: II. Follow-up of Affected Workers by Questionnaire, Spirometry, and Assessment of Bronchial Responsiveness 18 to 24 Months after Exposure Ended. Occup. Environ. Med.; 51-228; 1994.

16. Zika, R.G.; Moore, C.A.; Gidel, L.T.; Cooper, W.J. Sunlight-Induced Photodecomposition of Chlorine Dioxide. Chemical Engineering Impact and Health Effect Proceedings and Conference, 5th Meeting, Wittam, Va. pp. 1041-1053; 1984.

17. Cosson, H.; Ernst, W.R. Photodecomposition of Chlorine Dioxide and Sodium Chloride in Aqueous Solution by Irradiation with Ultraviolet Light. Ind. Eng. Chem Res 33:1468-1475, 1994.

18. Karpel, N.; Leitner, V.; Dore, M. Photodecomposition of Chlorine Dioxide by U.V. Irradiation. Part II, Kinetic Study. Wat. Res.; 26(12):1665-1672; 1992.

19. Southeastern Branch, NCASI. Evaluation of Toxilog Chlorine Personal Monitoring. Gainesville, Fl., May 1994.

20. Southeastern Branch. NCASI. Results of Preliminary Evaluation of Envirometrics Air-Chem Technologies Monitoring System for Chlorine and Hydrogen Sulphate. Gainesville, Fl., May 1994.

Lawrence Svirchev and George Astrakianakis are with the British Columbia Cancer Agency and Workers’ Compensation Board of British Columbia, Prevention Division, Vancouver, BC; Pierre Band is with the British Columbia Cancer Agency and the Health Protection Branch, Health Canada; Joel Bert is with the Health Protection Branch, Health Canada; Robert Janssena and Nhu Le are with the Workers’ Compensation Board of British Columbia; Clement Tang is with the British Columbia Cancer Agency and the Department of Chemical Engineering, University of British Columbia.