U.S. CHEMICAL SAFETY AND HAZARD INVESTIGATION BOARD

INVESTIGATION REPORT 

CHLORINE RELEASE

(16 Medically Evaluated, Community Evacuated) 

DPC ENTERPRISES, L.P. 

GLENDALE, ARIZONA

NOVEMBER 17, 2003 

KEY ISSUES: 

• MATCHING SAFEGUARDS TO RISK

• OPERATING PROCEDURES

• REACTIVE HAZARDS

• EMERGENCY RESPONSE

REPORT NO. 2004-02-I-AZ 

FEBRUARY 2007
DPC Glendale, AZ February 2007 

Contents 

1.0 INTRODUCTION ...........................................................................................................................8

1.1 Background.........................................................................................................................8

1.2 Investigative Process...........................................................................................................8

1.3 Characteristics of Chlorine .................................................................................................9

2.0 DPC ENTERPRISES, L.P. ............................................................................................................10

2.1 Corporate Structure...........................................................................................................10

2.2 Glendale Site.....................................................................................................................11

2.3 Process Description...........................................................................................................12

3.0 INCIDENT DESCRIPTION..........................................................................................................19

3.1 Incident .............................................................................................................................19

4.0 INCIDENT ANALYSIS................................................................................................................21

4.1 Operating Practice versus Procedure ................................................................................21

4.2 Scrubber Operating Procedure and Training ....................................................................23

4.3 Hazard Assessment and Control .......................................................................................24

4.4 Emergency Management ..................................................................................................30

4.5 Industry Guidance on Bleach Over-Chlorination .............................................................34

4.6 Regulatory Background ....................................................................................................35

5.0 ROOT AND CONTRIBUTING CAUSES....................................................................................38

5.1 Root Cause........................................................................................................................38

5.2 Contributing Causes..........................................................................................................38

6.0 RECOMMENDATIONS...............................................................................................................40

7.0 REFERENCES ..............................................................................................................................45

2
DPC Glendale, AZ February 2007 

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APPENDIX A CHLORINE PHYSICAL PROPERTIES AND EXPOSURE LIMITS......................47

APPENDIX B BLEACH OVER-CHLORINATION CHEMISTRY .................................................49

APPENDIX C DPC – GLENDALE INCIDENT TIME LINE...........................................................53

APPENDIX D DPC – GLENDALE INCIDENT EVENT TREE.......................................................54
DPC Glendale, AZ February 2007 

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Figures 

Figure 1. Surrounding community (Glendale and Phoenix, Arizona) .......................................................12

Figure 2. DPC-Glendale plot plan..............................................................................................................13

Figure 3. Chlorine bulk road trailer loading and caustic scrubber systems ...............................................14

Figure 4. Chlorine bulk road trailer ...........................................................................................................15

Figure 5. Caustic scrubber (typical)...........................................................................................................17

Figure 6. Hazardous chlorine concentration distances...............................................................................25

Tables 

Table 1. Health effects of acute chlorine exposure ......................................................................................9

Table 2. Oxidation reduction potential (ORP) alarm setpoints and actions...............................................18

Table 3. Safeguard reliability.....................................................................................................................26

Table 4. Voluntary safety management systems........................................................................................30
DPC Glendale, AZ February 2007 

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Acronyms and Abbreviations 

AIChE American Institute of Chemical Engineers 

CCPS Center for Chemical Process Safety 

CFR Code of Federal Regulations 

CSB U.S. Chemical Safety and Hazard Investigation Board 

EPA U.S. Environmental Protection Agency 

EPCRA Emergency Planning and Community Right-to-Know Act 

ERPG Emergency Response Planning Guideline 

HAP Hazardous Air Pollutant 

IDLH Immediately Dangerous to Life or Health 

mV Millivolts 

NIOSH National Institute for Occupational Safety and Health 

OSHA Occupational Safety and Health Administration 

ORP Oxidation Reduction Potential 

PEL Permissible Exposure Limit 

PHA Process Hazard Analysis 

PPE Personal Protective Equipment 

ppm parts per million 

psi pounds per square inch 

PSM Process Safety Management (OSHA) 

RMP Risk Management Program (EPA) 

REL Recommended Exposure Limit 

STEL Short-Term Exposure Limit 

vSMS Voluntary Safety Management System 
DPC Glendale, AZ February 2007 

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EXECUTIVE SUMMARY 

On November 17, 2003, a chlorine gas release at DPC Enterprises (DPC) in Glendale, Arizona, led to the 

evacuation of 1.5 square miles of Glendale and Phoenix. Five residents and 11 police officers sought 

medical attention for symptoms of chlorine exposure and were treated and released. 

The DPC Enterprises facility in Glendale repackages chlorine from railcars into smaller containers. DPC 

captures chlorine vented from these operations in one of two caustic scrubbers that also produce 

household bleach for sale as a byproduct. 

The U.S. Chemical Safety and Hazard Investigation Board (CSB) determined that excess chlorine vented 

to the scrubber, where it completely depleted the active scrubbing material (caustic) and over-chlorinated 

the scrubber. The resulting bleach decomposition reaction released a cloud of toxic gases into the 

surrounding community. Emissions continued at a decreasing rate for about six hours. The incident 

ended when workers injected additional caustic into the scrubber to stop the decomposition reaction. 

The CSB investigation identified the following root cause: 

• The single, procedural safeguard provided by DPC was not commensurate with the risk 

of over-chlorinating the scrubber. Additional safeguards should have been in place to 

prevent or mitigate scrubber over-chlorination, such as automatic shut-off of chlorine 

prior to over-chlorination, automatic or remote caustic injection to interrupt the 

decomposition reaction, or a downstream (secondary) scrubber to treat emissions from 

the over-chlorinated scrubber. 

The CSB investigation identified the following contributing causes: 

• Contrary to procedure, practice at the DPC site was to continue chlorine flow to the 

scrubber during quality control testing. Management did not detect this deviation. 
DPC Glendale, AZ February 2007 

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• Organizational and training issues in the Glendale Police and Fire Departments 

contributed to 11 Glendale police officers being exposed to chlorine. 

• Published guidance on scrubber over-chlorination provided no specific information on 

the composition, quantity, or duration of emissions expected during over-chlorination 

incidents, delaying stabilization of the scrubber and extending the duration of the 

incident. 

This CSB report makes recommendations to DPC Enterprises, the Glendale Fire and Police Departments, 

Maricopa County, and The Chlorine Institute. 
DPC Glendale, AZ February 2007 

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1.0 Introduction 

1.1 Background 

At about 11:30 a.m. on November 17, 2003, an uncontrolled decomposition reaction in a batch scrubber 

released chlorine gas into the air at the DPC Enterprises, L.P. (DPC) chlorine repackaging facility in 

Glendale, Arizona. Hazardous emissions continued for about six hours. Residents and workers in a 1.5 

square mile zone were told to evacuate, and 11 police officers and five members of the community sought 

medical treatment for exposure to chlorine. 

Because of the serious nature of this incident, which followed a large scale chlorine release from a DPC 

facility in Festus, Missouri, in 2002,1

 the U.S. Chemical Safety and Hazard Investigation Board (CSB) 

launched an investigation to determine root and contributing causes, and to make recommendations to 

help prevent similar incidents. The Industrial Commission of Arizona (State OSHA program); U.S. 

Environmental Protection Agency (EPA); and the Maricopa County Environmental Services Air Quality 

Division also investigated. 

1.2 Investigative Process 

The CSB investigators arrived at the DPC Glendale facility one day after the incident. The CSB 

interviewed DPC employees and emergency responders, reviewed company documents, consulted 

scientific publications and experts, and examined physical evidence. The investigation focused on DPC’s 

operating procedures and practices, its hazard assessment process, and its application of safeguards to 

prevent or mitigate reactive hazards. The CSB held a community meeting on June 9, 2004, in Glendale, 

1

 DPC Enterprises had a chlorine release of 48,000 pounds at its Festus, Missouri, site on August 14, 2002. CSB Report No. 

2002-04-I-MO, issued May 2003 and available at www.csb.gov, describes the CSB’s findings and recommendations for the 

Festus incident. 
DPC Glendale, AZ February 2007 

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Arizona, to update the community on the preliminary findings of the investigation and gather input from 

the emergency responders, community leaders, and the public. 

1.3 Characteristics of Chlorine 

1.3.1 Health Hazards of Chlorine 

Chlorine, a powerful oxidizer, is so highly toxic that it was used as a poison gas in World War I. 

Chlorine attacks the lungs, causing inflammation (pneumonitis) and fluid accumulation (pulmonary 

edema), and is intensely irritating to the eyes; prolonged and/or acute exposure may be fatal. Table 1 

summarizes typical symptoms of exposure to various concentrations of chlorine.2

Concentration 

(ppm in air) 

Health Effects 

1-3 ppm Mild mucous membrane irritation 

5-15 ppm Upper respiratory tract irritation 

30 ppm 

Immediate chest pain, vomiting, 

shortness of breath (dsypnea) and 

cough 

40-60 ppm 

Inflammation of lung tissues (toxic 

pneumonitis) and fluid accumulation 

(pulmonary edema) 

430 ppm Death within 30 minutes 

1,000 ppm Death within a few minutes 

Table 1. Health effects of acute chlorine exposure3

Because chlorine releases can produce effects toxic to humans, animals, and plants at considerable 

distances, identifying and controlling possible emission sources is extremely important. 

 

2

 Government agencies, including the Occupational Safety and Health Administration (OSHA) and the National 

Institute for Occupational Safety and Health (NIOSH), and industrial hygiene associations, including the American 

Industrial Hygiene Association (AIHA), have established exposure limits for chlorine. Appendix A, Table 2A 

documents selected limits. 
DPC Glendale, AZ February 2007 

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1.3.2 Physical Properties 

Chlorine is a greenish-yellow gas 2.5 times heavier than air at normal pressure and temperature. Chlorine 

releases usually stay close to the ground and dissipate relatively slowly. See Appendix A for additional 

physical properties of chlorine. 

1.3.3 Manufacture and Uses of Chlorine 

Manufacturers produced 12.5 million tons of chlorine in the United States in 2002.4 Chlorine is used to 

disinfect drinking water, and in the manufacture of bleach, paper, pesticides, solvents, medicines, and 

plastics, such as polyvinyl chloride (PVC). 

Chlorine is shipped as a liquid under pressure at ambient temperature. Large users may receive chlorine 

in railcar (90 ton) quantities. Smaller users typically receive chlorine in 150-pound cylinders, 1-ton 

containers, or 17-ton bulk road trailers. 

2.0 DPC Enterprises, L.P. 

2.1 Corporate Structure 

DPC Enterprises, L.P., is privately held and owns and operates six chlorine repackaging facilities. The 

company employs 50, including nine at the Glendale site. Publicly available sources and the company 

website indicate that DPC Enterprises is part of a family of companies, the DX Group5

 headquartered in 

Houston, Texas, with interests in organic chemicals manufacturing, oil well drilling additives, chemical 

distribution, and other businesses. 

3

 Ellenhorn and Barceloux, 1988. 

4

 Source – The Chlorine Chemistry Council. 

5

 From http://www.dxgroup.com 
DPC Glendale, AZ February 2007 

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A centralized group in Houston provides management, engineering, health, safety, environmental, and 

security services to both DPC Enterprises and DPC Industries. Services include developing standard 

operating procedures (SOPs) and related training materials, and coordinating regulatory compliance 

activities, including those related to OSHA (Process Safety Management) and EPA (Risk Management 

Program) process safety regulations. 

2.2 Glendale Site 

Chlorine operations in Glendale, Arizona, were established by McKesson in 1965. Van Waters & Rogers 

(VWR) bought the facility in 1986. DPC Enterprises, L.P. acquired the site from VWR in 1999 and 

subsequently upgraded the facilities. 

The surrounding community includes residential areas to the northeast and southwest, the Andalucia 

Elementary School, Maryvale Hospital, and a variety of retail businesses (Figure 1). Camelback Road 

and Grand Avenue are heavily traveled local roads. Glendale is a city of 234,000 (2003 estimate) on the 

west side of the Phoenix metropolitan area (Maricopa County). 
DPC Glendale, AZ February 2007 

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Figure 1. Surrounding community (Glendale and Phoenix, Arizona) 

2.3 Process Description 

At the site, DPC receives liquid chlorine in railcars and repackages it into smaller containers to distribute 

to local customers, and also manufactures sodium hypochlorite (or bleach). Figure 2 is a plot plan of the 

site, and shows the location of the major equipment involved in the November 17, 2003, incident. The 

caustic scrubbers used to control chlorine emissions are located in the southwest section, adjacent to the 

chlorine railcar unloading and bulk road trailer loading area. The chlorine building contains cylinder 

loading and bleach manufacturing facilities. 
DPC Glendale, AZ February 2007 

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Figure 2. DPC-Glendale plot plan 

2.3.1 Bulk Road Trailer Loading 

About once a month, DPC supplies a bulk road trailer of chlorine to a local municipal water treatment 

facility. To transfer the chlorine, hoses specifically designed for chlorine service connect the chlorine 

railcar and the bulk road trailer (Figure 3 and Figure 4) to the transfer piping system. Remotely operated 

valves on each end of the hoses shut off chlorine flows in an emergency.6

 The railcar initially contains 

 

6

 The chlorine release at the DPC Enterprises site in Festus, Missouri, resulted from the rupture of a transfer hose 

inadvertently fabricated using non-chlorine resistant materials, and the failure of remotely operated emergency 

valves to close. For a full discussion, visit the CSB website at www.csb.gov and download report 2002-04-I-MO. 
DPC Glendale, AZ February 2007 

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liquid chlorine with a mixture of chlorine vapor and air in the headspace; the trailer usually contains air 

but little or no liquid chlorine. 

Figure 3. Chlorine bulk road trailer loading and caustic scrubber systems 

Compressed air from the plant air supply pressurizes the headspace of the railcar and forces liquid 

chlorine to flow through the chlorine liquid transfer hoses and line into the bulk road trailer. As the bulk 

trailer fills, displaced chlorine vapors and air vent to one of two scrubbers. 
DPC Glendale, AZ February 2007 

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Figure 4. Chlorine bulk road trailer 

2.3.2 Scrubber Operation 

The Glendale scrubbers have two purposes: 

1. To capture (scrub) chlorine vented from repackaging operations (to protect workers and the 

public from exposure to chlorine). 

2. To produce saleable bleach (sodium hypochlorite solution) for distribution to local industrial and 

commercial customers. 
DPC Glendale, AZ February 2007 

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Because the optimum operating conditions for these two purposes are not identical,7

 operating the 

scrubbers simultaneously as critical safety devices and as batch bleach production units requires great 

care, and can greatly increase the risk of toxic releases. 

Chemistry 

Inside the scrubber, chlorine contacts a sodium hydroxide (caustic soda, NaOH) solution at a controlled 

temperature. The resulting reaction removes the chlorine and produces bleach (sodium hypochlorite, 

NaOCl); common salt (NaCl) is produced as a byproduct and remains with the bleach as a harmless 

impurity. Complete depletion of the caustic eliminates the scrubber’s ability to capture chlorine. 

Moreover, depletion also initiates a rapid decomposition of the bleach, referred to in the bleach industry 

as “over-chlorination,” which can release toxic chlorine compounds into the air (Appendix B). 

Design and Control 

The two Glendale scrubbers are 4,000 gallon, fiberglass reinforced plastic tanks (Figure 5).They operate 

as batch chemical reactors, with one unit receiving chlorine (the online scrubber), and the other operating 

as a backup (the standby scrubber).8

 Operators initially fill a scrubber with an aqueous solution 

containing 21 percent caustic.9

 Chlorine vented from repackaging operations is fed to the scrubber until 

the caustic concentration reaches 0.2-0.5 percent, as required by customer specifications for bleach.10 

The chlorine flow is then manually switched to the standby unit, the product bleach transferred to storage, 

and the depleted scrubber charged with fresh caustic solution. 

7

 Scrubbing efficiency is best at caustic concentrations above 8 percent; commercial specifications for bleach require 

much lower caustic concentrations. 

8

 The scrubbers are arranged in parallel–one unit cannot treat the gases vented from the other. 

9

 The 21 percent caustic solution yields the desired concentration of product bleach. 

10 Chlorine can also be fed directly to the scrubbers to complete a batch or when demand for bleach is high. 
DPC Glendale, AZ February 2007 

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As Figure 3 shows, a centrifugal pump circulates the scrubber solution through monitoring equipment and 

a heat exchanger (cooler) to the top of the scrubber where it mixes with chlorine vapors.11 The cleaned 

air vents through the top of the scrubber. Scrubber efficiency is normally close to 100 percent. 

Figure 5. Caustic scrubber (typical)

Two oxidation reduction potential (ORP)12 meters located in the suction line to the pump (Figure 3) track 

the concentration of caustic in the scrubber liquid. The meter readings are displayed in millivolts (mV) 

 

11 The scrubbers use venturi contactors, which maintain a slight vacuum on the chlorine vent lines to reduce leaks to 

atmosphere. 

12 Oxidation-reduction potential measures a solution’s ability to oxidize (accept electrons from) materials. Sodium 

hypochlorite is an oxidizer, and ORP measurements can be used to approximate the increase in bleach and 

corresponding decrease in caustic concentrations as the caustic reacts with chlorine. 
DPC Glendale, AZ February 2007 

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on a local panel (Figure 5). The ORP meter readings increase as the residual caustic concentration in the 

scrubber solution decreases. The correlation between ORP readings and caustic concentration is normally 

highly repeatable, but can be affected by a variety of factors, such as temperature, fouling, and the initial 

caustic concentration (see section 4.1). Each ORP meter is equipped with two alarms to help operators 

track the depletion of caustic in the bleach batch (Table 2); however, no automated control actions occur 

based on the ORP meters’ outputs. 

Alarm Setpoint 

Value (millivolts) DPC Operator Action Required 

First ORP Alarm 

(Process Meter) 

500 mV 

(approx 1.5% 

excess caustic) 

• Acknowledge alarm 

• Remain in area 

• Sample and perform laboratory analysis at 15-minute intervals 

Second ORP Alarm 

(Process Meter) 

515 mV 

(approx 1.35% 

excess caustic) 

No action specified 

Third ORP Alarm 

(Safety Meter) 

530 mV 

(approx 1% excess 

caustic) 

• Acknowledge alarm 

• If venting at a high rate: 

o Stop chlorine flow to scrubber 

o Sample and perform laboratory analysis at 5-

minute intervals 

Fourth ORP Alarm 

(Safety Meter) 

545 mV 

(approx 0.66% 

excess caustic) 

No action specified 

Stop scrubbing operations when excess caustic between 0.2–0.5% 

based on laboratory analysis 

Table 2. Oxidation reduction potential (ORP) alarm setpoints and actions 
DPC Glendale, AZ February 2007 

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A batch of bleach is complete when laboratory (off-line) analysis of the scrubber solution indicates that 

the residual caustic concentration meets DPC customer specifications of 0.2-0.5 percent.13

3.0 Incident Description 

3.1 Incident 

On November 17, 2003, DPC personnel were transferring chlorine from a railcar to a bulk road trailer 

when the scrubber became over-chlorinated and began releasing chlorine to the atmosphere. 

At approximately 7:00 that morning, in preparation for the chlorine transfer, operators recorded the 

Oxidation Reduction Potential (ORP) meter reading of 490 mV and tested the solution in the scrubber. 

They measured a caustic concentration of 1.60 percent caustic (by weight), indicating that the scrubber 

had not yet reached its target concentration of 0.2-0.5 percent. 

Shortly after 9:00 a.m., the operators began transferring chlorine to the bulk road trailer.14 Air and 

chlorine vapors from the trailer flowed to the scrubber (Figure 3), reducing the caustic concentration. 

Operators continued working on other assigned tasks.15 At 10:00 a.m., operators recorded an ORP 

reading of 510 mV, again tested the scrubber’s contents, and recorded the caustic concentration at 

1.18 percent. 

According to the operators, the first safety alarm on the caustic scrubber, set at an ORP reading of 

530 mV, activated at approximately 10:15 a.m. An operator pressed the acknowledge button to silence 

the alarm, checked the ORP value, and returned to other tasks. 

13 The caustic concentration in the scrubber solution is determined by laboratory analysis to evaluate the remaining 

capacity of the scrubber to react with chlorine vapors. 

14 The operators tested emergency shutdown systems and conducted leak checks before starting the chlorine transfer. 
DPC Glendale, AZ February 2007 

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Shortly after 11:00 a.m., the second safety alarm, set at an ORP reading of 545 mV, activated. An 

operator pressed the alarm acknowledge button to silence the alarm and went to get a container for a 

scrubber solution sample. Upon returning to the scrubber area, the operator heard rumbling and saw 

liquid splashing from, and a green cloud forming around, the scrubber. 

3.1.1 Emergency Shutdown/Facility Evacuation 

The operator instructed nearby personnel to evacuate and pushed an emergency shutdown button,16 which 

closed automatic valves on the loading line and the scrubber vent line connected to the bulk road trailer 

(Figure 3). He activated the plant’s emergency alarm, and evacuated with other DPC employees to the 

designated assembly area. DPC’s plant manager called 911 and then telephoned neighboring businesses 

to inform them of the release. 

3.1.2 Emergency Response 

The Phoenix Fire Department was first to arrive on the scene, and were joined by the Glendale and 

Tempe fire departments and the Glendale and Phoenix police departments. 

Responders established initial boundaries for the potentially hazardous area, and later expanded the 

boundaries when plume modeling by the Tempe Fire Department indicated that the potentially hazardous 

area could be larger. 

Authorities used an automated telephone call-down system and media announcements to notify the 

community in the potentially hazardous area to evacuate. Police officers also drove through the 

evacuation area and used their public address systems to notify residents. None of the officers who 

entered the potentially hazardous area wore respiratory protection. The evacuated area included about 

15 One operator filled drums of bleach, while another transferred 50-weight percent caustic solution from a railcar to 

a storage tank, and moved bleach drums to storage. 

16 Testing conducted by the CSB after the incident verified that the emergency shutdown system operated properly. 
DPC Glendale, AZ February 2007 

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2,500 homes with 7,200 residents, and several large businesses (Figure 1). Students at the Andalucia 

Elementary School sheltered-in-place.17 

Responders set up water sprays to absorb chlorine gas, and entered the site at approximately 1:30 p.m. to 

close manual valves associated with the railcar, bulk road trailer, and scrubber. Phoenix Fire Department 

responders measured chlorine concentrations of 20-35 parts per million (ppm) close to the scrubber, with 

higher spikes when gases periodically vented. The rate of venting eventually decreased, and all evacuees 

were allowed to return to their homes about four and one half hours after the over-chlorination of the 

scrubber. 

Minor venting of chlorine from the scrubber continued until DPC personnel added caustic to the scrubber 

to stabilize the contents and absorb any remaining chlorine. No further emissions were detected. 

As a result of the incident, 11 police officers and five citizens were evaluated for symptoms consistent 

with chlorine exposure. 

4.0 Incident Analysis 

4.1 Operating Practice versus Procedure 

Bleach manufacturing practice18 at the DPC Glendale site deviated significantly from the written SOPs 

when chlorine vented to the scrubber at a high rate. 

DPC’s written bleach production SOPs required that the chlorine flow to the scrubber be shut off and that 

the scrubber solution be sampled at five-minute intervals when the Oxidation Reduction Potential (ORP) 

17 To shelter-in-place is to remain indoors while restricting the ability of toxic substances to enter by turning off 

ventilating systems, moving to interior rooms, and sealing openings. 

18 Practice is how operators actually perform a task. Procedure is how the SOP specifies performing that task. 
DPC Glendale, AZ February 2007 

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meter reading reached 530 mV and chlorine was venting to the scrubber at a high rate. DPC management 

considered bulk road trailer loading to produce a high rate of venting. 

In practice, however, operators continued the flow of chlorine to the scrubber until the target 

concentration was reached, while periodically sampling the scrubber solution. On the day of the incident, 

the scrubber over-chlorinated while the operator was preparing to take a sample for laboratory analysis. 

Several characteristics of the DPC process made the scrubber susceptible to over-chlorination: 

• At the target concentration of 0.2-0.5 percent caustic, only 1-2 percent of the initial caustic charge 

remained, leaving little reserve to protect the scrubber in case of changes in chlorine flow rate or 

delays in operator response near the end of a batch. 

• Chlorine flow to the scrubber varied greatly. Based on production log entries, CSB investigators 

calculated that the flow of chlorine gas from the bulk road trailer to the scrubber at least tripled 

toward the end of the transfer on the day of the incident.19

• The ORP meter readings were susceptible to errors due to a variety of factors, including 

temperature swings, changes in the initial caustic concentration, variation in the chemistry of the 

water used to prepare the caustic solution, sensor fouling, and installation-specific factors. These 

potential error sources would similarly affect both ORP probes. 

These factors combined to make the time between sounding of the final ORP alarm and over-chlorination 

both variable and difficult to predict. Together with the operating practice of maintaining chlorine flow to 

the scrubber while sampling, this greatly increased the risk of scrubber over-chlorination. 

19 The flow of chlorine likely tripled because the gas initially vented from the trailer contained appreciable amounts 

of nitrogen. As the trailer filled with liquid, the vented gas became chlorine enriched. 
DPC Glendale, AZ February 2007 

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In addition, DPC management failed to recognize that practice deviated from the written bleach 

production SOP. 

4.2 Scrubber Operating Procedure and Training 

The bleach production (scrubber) SOP did not reflect the sensitivity of the process to over-chlorination. 

Furthermore, it did not provide operators with key information about the consequences of deviating from 

operating limits: 

• The SOP warned that relying on the ORP meters to determine excess caustic could result in overchlorination with the “possible” release of chlorine, and directed operators to verify ORP readings 

using laboratory measurements. However, it did not indicate clearly why or how the ORP 

readings could vary or that an incident with potentially serious off-site safety and environmental 

consequences could result. 

• The SOP specified no actions to be taken upon receipt of the fourth (final) alarm (such as doublechecking that the chlorine flow was shut off before sampling), and contained no warning that the 

time between this alarm and over-chlorination could be brief. 

• The SOP did not document which operations produced high rates of chlorine venting, and thus 

required more conservative operation of the scrubber. As a result, the operators were unaware 

that bulk road trailer loading was considered to be a high vent rate operation. 

• The SOP was available for employee review, but was not routinely used in daily operation. The 

operators stated that they were unfamiliar with all the requirements of the SOPs. 

Operator training, based on the operating procedure, did not address the sensitivity of the scrubber to 

over-chlorination or the safety and environmental consequences of over-chlorination. 
DPC Glendale, AZ February 2007 

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Operators are far more likely to follow procedures when they understand why and under what 

circumstances specific actions are required (CCPS, 1994, 1996). Operating procedures for hazardous 

processes thus need to provide clear guidance on the consequences of deviation and the steps needed to 

correct or avoid such deviations [§OSHA 1910.119(f)(ii)], and on any special circumstances that require 

changes to normal practice. Managers also need to monitor actual practice to ensure that procedures are 

followed. 

4.3 Hazard Assessment and Control 

The November 17 chlorine release was serious, and had the potential to significantly harm workers and 

the community. The CSB investigators estimate that the scrubber could have released up to 1,920 pounds 

of chlorine (Appendix B). Fortunately, the weather conditions during the incident were favorable for 

dissipation of the release. Under these conditions, the CSB estimates that hazardous concentrations20 of 

chlorine likely extended out as far as 0.4 miles from the site.21 A similar release under highly stable 

atmospheric conditions could produce toxic concentrations of chlorine up to 1.3 miles from the DPC site. 

The areas of Glendale and Phoenix within these distances of DPC are shown in Figure 6. Approximately 

750 people live inside the smaller (0.4 mile radius) circle (Region 1), while nearly 30,000 live inside the 

larger (1.3 mile radius) area (Region 2). Depending on the wind direction and atmospheric conditions, a 

1,920 pound release in this densely populated area could place many people at risk. 

20 Based on reaching chlorine’s Emergency Response Planning Guideline (ERPG)-2 concentration of 3 ppm at the 

distances given. Concentrations closer to the DPC site would have been higher. The ERPG-2 concentration is the 

maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without 

experiencing irreversible or other serious health effects, or symptoms that could impair an individual’s ability to 

take protective action (American Industrial Hygiene Association). The EPA and other organizations use ERPG-2 

concentrations in emergency response planning. Exposure to lower concentrations of chlorine can still cause 

symptoms, see Table 1. 

21 The release occurred during the day and with moderate winds, conditions that favored rapid dispersion of the 

release. Highly stable atmospheric conditions, such as often occur at night, could slow dispersion and increase the 

toxic endpoint distance. 
DPC Glendale, AZ February 2007 

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Figure 6. Hazardous chlorine concentration distances 

4.3.1 Matching Safeguards to Risk 

DPC relied on a single administrative safeguard to prevent scrubber over-chlorination: an SOP. While 

SOPs are essential in any process safety program, such procedures are regarded as the least reliable form 
DPC Glendale, AZ February 2007 

26

of safeguard in preventing process incidents. The Center for Chemical Process Safety (CCPS) (2004) has 

ranked safeguards in decreasing order of reliability: 

Reliability Type Examples 

Most Reliable Passive Safeguards 

• Reduced inventory of hazardous substances 

• Use of chemistry with reduced toxicity 

Less Reliable Active Safeguards • Emergency shutdown systems 

• Downstream (secondary) scrubbers 

Least Reliable Procedural 

Safeguards • Operating procedures 

Table 3. Safeguard reliability22

Passive safeguards, such as reduced inventory of hazardous substances, cannot readily fail, but, as in this 

case, are not always feasible. Active safeguards, such as emergency shutdown systems, must be 

maintained and tested, and may suffer from shared (common mode) failure mechanisms such as the loss 

of utilities, making them potentially less reliable than passive safeguards. Procedural safeguards, such as 

SOPs, rely on personnel consistently making correct and timely decisions while performing other duties, 

and potentially while stressed or fatigued. Procedural safeguards are thus considered to be the least 

reliable of the three types. 

Failures with potentially severe consequences, such as a chlorine release in a densely populated area like 

Glendale, may require multiple, independent safeguards, in addition to procedures that, in aggregate, have 

the effectiveness and reliability needed to prevent, control, or mitigate the consequences of critical 

failures. 

 

22 CCPS, 2004. 
DPC Glendale, AZ February 2007 

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Examples of active safeguards that could reduce the likelihood or reduce the consequences of scrubber 

over-chlorination include (but are not limited to): 

• Automatic shutoff of the chlorine upon high ORP alarm to prevent over-chlorination.23 

• A downstream scrubber to treat the gases released by over-chlorination. The standby scrubber 

could be configured for this or a dedicated emergency scrubber installed. 

• Automatic or remote injection of caustic into an over-chlorinated scrubber, which could stabilize 

the scrubber quickly and prevent the extended release of toxic materials.24

Additional procedural safeguards, such as stopping the chlorine feed to the scrubber at a higher caustic 

concentration and completing the bleach batch in Glendale’s continuous bleach manufacturing system, 

could also reduce the likelihood of over-chlorination, but should be combined with active safeguards to 

reliably protect against the consequences of over-chlorination. 

In addition, methods such as Layers of Protection Analysis (LOPA) have been developed that can help 

companies assess if their safeguards will effectively and reliably control serious hazards (CCPS, 2001). 

Chlorine scrubbers, which are batch reactive systems with high-consequence failure modes, are good 

candidates for evaluation using LOPA. 

4.3.2 Process Hazards Analysis 

Hazards at chemical facilities are usually identified, and their potential for causing harm estimated, in a 

Process Hazard Analysis (PHA). DPC performed a PHA of the Glendale chlorine system in 1999 when 

23 Stopping chlorine flow to the scrubber after over-chlorination has begun will not stop the bleach decomposition 

reaction, although it will reduce the emission of unscrubbed chlorine. 

24 All these measures are used at other bleach manufacturing facilities in the US, according to a survey by The 

Chlorine Institute, and may be considered best practices. 
DPC Glendale, AZ February 2007 

28

 

DPC had just acquired the site, and another in 2001 when the company installed a continuous bleach 

production process. While the PHAs evaluated a variety of equipment failure mechanisms, they did not 

review the scrubber operating procedure and did not directly address failure to turn off the chlorine flow 

to the scrubber at the end of a batch.25 

DPC estimated that the scrubber released 3,500 pounds of chlorine during the November 17, 2003, 

incident, a quantity that could cause serious off-site consequences (see section 4.3).26 DPC could and 

should have made this estimate as part of its risk assessment process before the incident and taken steps to 

reduce the likelihood or severity of scrubber over-chlorination. 

While the “What If?” checklist PHA method DPC used for both studies is a recognized approach, relying 

on checklists can impede the identification of unusual or not previously recognized hazards. Good 

practice is to use a variety of methods when revalidating PHAs for highly hazardous processes, as using 

different PHA methods will, over time, provide a more complete assessment of hazards. 

The Glendale PHAs did not identify and address the known scrubber failure mode of over-chlorination. 

Companies should review their chlorine scrubber PHAs to ensure that scenarios potentially leading to 

over-chlorination have been identified and reviewed, and that adequate safeguards are in place to control 

this serious hazard. Guidance for planning and conducting effective PHAs is provided in many CCPS 

publications (1995, 1999, 2001). 

25 DPC is required by OSHA Process Safety Management and EPA Risk Management Program regulations to 

perform or revalidate PHAs at specified intervals and after significantly changing the chlorine handling processes 

at the facility. 

26 DPC based its estimate of 3,500 pounds on the release of most of the chlorine fed to the scrubber. The CSB 

estimate of up to 1,920 pounds is an upper limit based on the chemistry described in Appendix B. 
DPC Glendale, AZ February 2007 

29

4.3.3 Process Safety Management Audits 

The Glendale site is covered by OHSA’s Process Safety Management (PSM) regulation (see section 

4.6.1). 29 CFR 1910.119 (o), “Compliance Audits” requires employers to “certify that they have 

evaluated compliance with the provisions [of the regulation] at least every three years to verify that the 

procedures and practices developed under the standard are adequate and are being followed.” These 

audits provide critical feedback and correction to maintain PSM program effectiveness. 

A manager from DPC’s corporate health, safety, environmental, and security group performed a PSMrequired audit of the Glendale facility in June 2002. The checklist-based one-day audit generated only 

eight recommended actions, including six that addressed documentation, and found that all procedures, 

training, and process safety information were up-to-date and accurate. 

This audit failed to detect the missing process safety information and scrubber operating procedure 

problems uncovered during the CSB’s investigation. For example, no Piping and Instrumentation 

Diagrams (P&IDs) existed for the site prior to the November 17, 2003, incident, although the PSM 

regulation specifically requires them. 

The audit did not rigorously examine the underlying PSM program elements; rather, the focus was on 

whether the PSM program procedures developed by the corporate support group were in place at the site. 

Moreover, the same corporate group performing the audit had also developed the site PSM program, 

written the site operating procedures, and participated in or led the site PHAs. The weaknesses in PSM 

program elements the CSB identified in its investigation are not readily detectable using such an audit 

approach. 

Companies can benefit by incorporating independent auditors into their safety program. Using multiperson audit teams can also lead to higher quality audits by providing a variety of insights into program 

elements and their implementation. The CCPS (1993) publishes guidelines that can help companies plan 

and perform effective audits. 
DPC Glendale, AZ February 2007 

30

4.3.4 Voluntary Safety Management Systems 

Voluntary safety management systems (vSMS) can provide access to state-of-the-art management 

practices, expert advice, a common framework for all sites, guidance on continuous safety improvement, 

and objective feedback on safety system implementation. Table 4 lists several notable examples of such 

programs. 

DPC Enterprises’ sites have not yet been verified and certified under a voluntary safety system. Their 

safety performance could benefit from such verification and certification. 

Program Sponsoring Organization (web page) 

Responsible DistributionTM National Association of Chemical Distributors (www.nacd.com) 

Responsible CareTM American Chemistry Council (www.responsiblecare.com) 

Voluntary Protection 

Program OSHA (www.osha.gov/dcsp/vpp) 

Table 4. Voluntary safety management systems 

4.4 Emergency Management 

4.4.1 DPC Emergency Planning and Response 

Prior to the incident, DPC Glendale provided local emergency responders with information on hazardous 

chemicals at its facility and on the company emergency response plan, as required by EPA’s Emergency 

Planning and Community Right-to-Know Act (EPCRA) and RMP regulations. 

Company personnel followed DPC’s emergency response plan during the incident. The operator 

activated the emergency shutdown system, shutting off the chlorine flow to the bulk road trailer and the 
DPC Glendale, AZ February 2007 

31

 

scrubber.27 He also activated the plant alarm system, after which DPC personnel evacuated the facility 

without mishap. The plant manager notified emergency responders and adjacent facilities of the release. 

In addition, plant management personnel remained on the scene. They provided responders with an 

estimated release quantity of 3,500 pounds of chlorine, and with information on valve locations to help 

firefighters ensure that the scrubber was isolated from all chlorine sources. 

4.4.2 Public Agency Emergency Response 

The responding fire departments rapidly activated a unified command structure, and established an 

incident command center near the DPC site.28,29 The Glendale, Phoenix, and Tempe fire departments 

contributed resources to the response. Fire department communications worked well throughout the 

incident. County and state agencies also responded and provided environmental monitoring and other 

assistance. 

The incident commander established an initial isolation zone covering roughly two city blocks, based on 

the DOT Emergency Response Guidebook’s recommendations for chlorine releases, and excluded traffic 

from a segment of Camelback Road (a major east-west roadway). Based on dispersion modeling by the 

Tempe Fire Department,30 the incident commander expanded the isolation zone to a 1 by 1.5 mile 

rectangle extending downwind from the DPC site (Figure 1). 

27 Post-incident testing witnessed by the CSB confirmed that the isolation valves operated properly and did not leak. 

Chlorine flow to the scrubber likely stopped within 1-2 minutes of the start of the incident. 

28 The responders followed a standard protocol for emergency response based on the National Incident Management 

System (NIMS), established through the Department of Homeland Security. See 

www.dhs.gov/dhspublic/interapp/press_release/press_release_0363.xml . 

29 Phoenix area fire departments share a common dispatch system and routinely provide emergency services across 

city lines. These departments also participate in a regional response plan through the Maricopa County 

Department of Emergency Services or the Arizona Division of Emergency Management. 

30 Tempe personnel were equipped with and trained on the use of the EPA CAMEO (Computer Aided Management 

of Emergency Operations) software. They used the ALOHA (Areal Locations of Hazardous Atmospheres) 

dispersion modeling program, included with CAMEO, to estimate the potential extent of the toxic cloud. 
DPC Glendale, AZ February 2007 

32

 

The incident commander, in consultation with fire department personnel,31 ordered the evacuation of 

most of the isolation zone and sheltering-in-place for students at the Andalucia Elementary School. City 

buses were used to transport residents to a refuge location southwest (upwind) of the evacuation zone, 

south of the Maryvale Hospital (Figure 1). 

Glendale and Phoenix police controlled access to the isolation zone and notified residents of the 

evacuation order using their squad car public address systems. The Glendale City and Maricopa County 

telephone call-down systems and the local media were also used to contact residents and inform them of 

the need to evacuate.32

Emergency responders suspected that chlorine might be leaking from the chlorine railcar because of the 

extended duration of the chlorine emissions. They closed manual valves on the railcar and bulk road 

trailer; however, emissions continued, albeit at a decreasing rate, until DPC personnel added caustic to the 

scrubber, stabilizing it. 

Air quality monitoring by the Arizona Department of Environmental Quality (ADEQ) continued until the 

scrubber was secured and all emissions had ceased. The incident commander closed the incident at 8:54 

p.m. 

The size of the release, the favorable weather conditions, and the emergency response efforts in this 

incident limited the community’s exposure to chlorine. Five residents exposed to low concentrations of 

chlorine were transported for medical evaluation, examined, and released. 

31 Shelter-in-place decisions can be complex, and involve balancing the potential hazard of remaining at the shelter 

location with being exposed to toxic material while attempting to evacuate. 

32 Technical and coordination issues with the call-down systems caused some confusion. Glendale residents 

received messages from both the City and County systems. The County system’s message began clearly in 

Spanish, but the volume then dropped, making the English portion of the message unintelligible. This led to the 

Glendale 911 center being inundated with calls. The Glendale system has since been shut down and replaced by 

the Maricopa County system. 
DPC Glendale, AZ February 2007 

33

 

4.4.2.1 Police Chlorine Exposure 

The Glendale Police Department provides its officers with air purifying respirators (APRs) designed to 

protect them from the effects of toxic gases. The 11 Glendale Police Department officers33 treated for 

chlorine exposure were not wearing their APRs when they were exposed because: 

• The incident command system did not deliver timely information about the location of chlorinecontaminated areas to the officers. This was due to poor integration of the police into the incident 

command structure and technical factors, including incompatible fire and police radio 

frequencies. Officers were not always aware they were entering a contaminated area. 

• Police dispatchers sent officers directly into the isolation zone without first directing them to a 

staging area where they could be briefed on incident conditions, review Glendale Police 

Department safety procedures for hazardous materials incidents, and check their personal 

protective equipment (PPE). As a result, some officers did not have their APRs with them. 

• Some officers carrying APRs failed to use them. They interpreted warnings from fire department 

personnel to mean that the police APRs offered no protection against chlorine, when, in fact, their 

APRs would have been highly effective.34

Failure to use PPE reflects a need for training beyond the officers’ First Responder–Awareness level. 

Officers’ duties during this incident included evacuating citizens from potentially chemically affected 

areas, making First Responder - Operations level training more appropriate.35 The exposed officers had 

33 Nine officers were taken to hospitals, evaluated, and released; two were evaluated at the scene and released. 

34 APRs are not permitted when hazardous materials are above the “Immediately Dangerous to Life or Health” 

(IDLH) concentration, which is 10 parts per million (ppm) for chlorine. Fire department responders were 

equipped with Self Contained Breathing Apparatus (SCBA), which are protective above the IDLH. Chlorine 

concentrations were above the IDLH close to the scrubber where fire department, but not where police, personnel 

were stationed. 

35 Training requirements are specified in OSHA 29 CFR 1910.120(q). 
DPC Glendale, AZ February 2007 

34

 

also not received their annual hazardous materials refresher training. Improved training would likely 

have increased APR use and reduced officer exposure to chlorine. 

An earlier release at the site in 1988 also exposed Glendale Police Department officers to chlorine. Police 

must be integrated into the incident command structure, given timely hazard information, briefed on the 

hazards they face, checked to ensure they are carrying their PPE, and trained to recognize and effectively 

respond to hazardous materials incidents. Periodic hazardous materials exercises are also essential to 

ensuring that the Glendale Fire Department’s and Glendale Police Department’s response to future 

hazardous materials incidents protects the well-being of both the public and responders.36

4.5 Industry Guidance on Bleach Over-Chlorination 

Scrubber over-chlorination is a documented hazard known to result in the release of toxic materials. To 

better understand the characteristics of scrubber over-chlorination, the CSB conducted an extensive 

technical literature search; reviewed guidance documents published by The Chlorine Institute (CI); and 

interviewed academic and industry experts. These sources generally agreed on the chemistry involved 

(Appendix B), but did not provide quantitative guidance on important features of over-chlorination 

incidents, such as: 

• The total amount of toxic gases emitted during a release due to bleach decomposition. 

• The identity of the major toxic materials released. While the assumption has been that it is 

chlorine, materials with different properties, such as hypochlorous acid, might also be released. 

• The duration of the release. 

36 Resources for planning, executing, and evaluating hazardous materials exercises include the National Response 

Team’s NRT-2 (1990) and the Department of Homeland Security’s Homeland Security Exercise and Evaluation 

Program (2006). 
DPC Glendale, AZ February 2007 

35

 

• The impact of variation in scrubber operating conditions on release characteristics. 

• Methods to control or mitigate over-chlorination events. 

This information is needed to accurately design mitigation equipment, such as downstream (secondary) 

scrubbers, and to provide better guidance to emergency responders. In this incident, better information 

about the characteristics of over-chlorination incidents would likely have led to an earlier decision to add 

caustic to the scrubber, reducing the duration and impact of the incident. 

The Chlorine Institute publishes guidance documents relevant to the design and operation of chlorine 

scrubbers used for bleach production,37 including “Chlorine Scrubbing Systems, Chlorine Institute 

Pamphlet 89” and “Sodium Hypochlorite Manual, Chlorine Institute Pamphlet 96.” These documents 

advise that over-chlorinating scrubbers is dangerous and can lead to the release of hazardous materials, 

including chlorine. However, the versions available at the time of the incident did not recommend 

specific safeguards to prevent, control, or mitigate the consequences of scrubber over-chlorination. The 

2006 edition of Pamphlet 89 provides useful recommendations for scrubber safeguards, but not all of 

these would be effective in preventing or stopping bleach decomposition due to over-chlorination. 

Public safety would benefit from additional guidance quantifying the consequences of scrubber overchlorination and providing more comprehensive recommendations for best practices to prevent these 

dangerous events. 

4.6 Regulatory Background 

The OSHA PSM and the EPA RMP regulations are both intended to reduce the risk of catastrophic 

releases of highly hazardous chemicals. PSM focuses on how releases impact workers, while RMP 

37 DPC’s corporate engineering and safety staff indicated that they refer to The Chlorine Institute’s publications for 

guidance. 
DPC Glendale, AZ February 2007 

36

 

incorporates the elements of PSM and adds requirements for evaluating off-site consequences and 

community outreach. Because the Glendale site contains greater-than-threshold quantities of chlorine 

under both PSM (1,500 pounds) and RMP (2,500 pounds), DPC has compliance programs for both 

programs. The caustic scrubber is also permitted by Maricopa County as an air pollution control device. 

4.6.1 The OSHA PSM Regulation 

The CSB’s investigation revealed significant weaknesses in the DPC Glendale PSM program, as 

discussed in Process Hazard Analysis-1910.119 (e) (Section 4.3.2); Operating Procedures–1910.119 (f) 

(Section 4.2); and Training–1910.119 (g) (Section 4.2).

4.6.2 The EPA RMP Regulation 

The RMP regulation requires facilities to submit information on the potential off-site consequences of 

their operations, including the distance at which toxic effects could occur in the most probable serious 

accident at the site. This distance, the alternative case toxic endpoint distance, was reported by DPC as 

0.6 miles for the Glendale site, close to the CSB-estimated distance for this incident.38

Based on the U.S. Census Bureau’s Landview 6 mapping software, approximately 3,300 people live 

within DPC’s alternative case distance. Thus, the most likely anticipated release scenario at the DPC 

facility would be expected to impact a large number of local residents. Approximately 7,200 live within 

the much larger area evacuated during this incident. 

38 The EPA also requires sites to report the worst case distance; in this case the complete discharge of a chlorine rail 

car in 10 minutes, under stable atmospheric conditions unfavorable for dispersion. For the Glendale site, this 

distance is 14 miles. Approximately 1.7 million people live within this radius of the DPC Glendale site. 
DPC Glendale, AZ February 2007 

37

 

4.6.3 Maricopa County Environmental Quality Division, Air Quality Department 

Under an EPA State Implementation Plan (SIP), Maricopa County administers the pollution control 

permit program in Glendale. The County permitted DPC as a non-major source of chlorine 

emissions.39,40 The permit required DPC to ensure that fill lines and hoses were vented “through a 

properly working scrubber that is maintained and operated in accordance with the approved operations 

and maintenance plan,” and to have and follow operating procedures to “minimize emissions from the 

transferring, handling, or repackaging” of chlorine. 

The operations and maintenance plan submitted by DPC and approved by the County specified daily 

logging of ORP meter readings from the scrubber, but not of the more reliable laboratory measurements 

of caustic concentration. 

39 Non-major sources emit less than 10 tons per year of any single Hazardous Air Pollutant (HAP) and less than 25 

tons per year of total HAPs. Chlorine was the only HAP permitted at the DPC Glendale site. 

40 No National Emissions Standard for Hazardous Air Pollutants (NESHAP) exists for chlorine. The EPA has 

determined that chlorine is not a persistent pollutant, in that it photolyzes rapidly to hydrochloric acid (HCl), a 

much less toxic substance, following release. The pollution control permit program is not designed to address 

major releases of highly hazardous materials, such as the November 17, 2003, DPC release. 
DPC Glendale, AZ February 2007 

38

5.0 Root and Contributing Causes 

5.1 Root Cause 

The safeguards provided on the DPC scrubber were not commensurate with the risk of over-chlorination. 

• DPC’s corporate standards relied solely on procedural safeguards against scrubber overchlorination. 

• DPC’s corporate hazard assessment process did not identify or address the consequences 

of failure to follow the bleach manufacturing SOP, including potential off-site 

consequences. 

• DPC’s internal PSM/RMP audit program did not detect deficiencies in operating 

procedures, training, operating practice, process safety information, and hazard 

assessment. 

5.2 Contributing Causes 

1. Practice at DPC’s Glendale site deviated from the scrubber SOP when chlorine was venting at a 

high rate, increasing the risk of scrubber over-chlorination. 

• DPC’s corporate scrubber SOP and training materials did not address the consequences 

of deviating from the scrubber SOP. 

• Compliance with the scrubber procedure was not enforced, further weakening an already 

inadequate safeguard. 

• Operators were inadequately trained on the consequences of over-chlorination and on the 

sensitivity of the process to over-chlorination. 
DPC Glendale, AZ February 2007 

39

2. Organizational and training problems contributed to the exposure of 11 Glendale Police 

Department officers to chlorine. 

• Inadequate integration of the Glendale Police Department into the incident command 

structure prevented the timely transmission of critical safety information to responding 

officers. 

• Deployment of Glendale Police Department officers into chlorine-impacted area without 

briefing or safety equipment checks allowed them to enter hazardous locations without 

APRs. 

• Inadequate hazardous material training led to Glendale Police Department officers not 

wearing their APRs. 

3. Published guidance on scrubber over-chlorination does not provide specific information on the 

composition, quantity, or duration of emissions expected during over-chlorination incidents. 
DPC Glendale, AZ February 2007 

40

6.0 Recommendations 

DPC Enterprises 

1. Establish and implement DPC corporate engineering standards that include adequate layers of 

protection on chlorine scrubbers at DPC facilities, including 

• additional interlocks and shutdowns, such as automatically stopping chlorine flow to the 

scrubber upon oxidation-reduction potential alarm; 

• mitigation measures, such as systems to automatically add caustic to over-chlorinated 

scrubbers, or back-up scrubbing capability to treat emissions from over-chlorinated 

scrubbers; 

• increases in the final caustic concentration in the scrubbers to eight percent or higher to 

provide a substantial safety margin against over-chlorination; and 

• use of the site’s continuous bleach manufacturing system to convert scrubber solution to 

saleable bleach. 

2. Revise scrubber SOPs to include: 

• clearly described operating limits and warnings about the consequences of exceeding those 

limits, and 

• the safety and environmental hazards associated with scrubber over-chlorination. 

3. Train employees on the revised SOPs and include a test to verify understanding. Periodically 

review operator understanding of and conformance to the scrubber SOPs. 
DPC Glendale, AZ February 2007 

41

4. Include scrubber operation in facility PHAs. Ensure that they: 

• include lessons learned from this incident and other DPC scrubber incidents, as well as 

industry experience with over-chlorination, and 

• consider off-site consequences when evaluating the adequacy of existing safeguards. 

5. Use a qualified, independent auditor to evaluate DPC’s PSM and RMP programs against best 

practices. Implement audit recommendations in a timely manner at all DPC chlorine repackaging 

sites. 

6. Implement a recognized safety management system, including third party verification and 

certification, to achieve documented continuous improvement in safety performance at Glendale 

and the other DPC chlorine repackaging sites. 

Glendale Fire Department 

1. Work with the Glendale Police Department to integrate them into the incident command structure 

during hazardous material incidents, and address communications issues, such as radio 

interoperability, to ensure the timely transmission of critical safety information to responding 

officers. 

2. Conduct hazardous materials exercises with the Glendale Police Department to identify and 

resolve police/fire integration issues. Coordinate exercise planning with the Arizona Division of 

Emergency Management Exercise Officer and with the Maricopa County LEPC. Schedule 

periodic hazardous materials incident drills to ensure safe and effective responses to future 

hazardous materials incidents. 
DPC Glendale, AZ February 2007 

42

Glendale Police Department 

1. Work with the Glendale Fire Department to integrate the Glendale Police Department into the 

command structure during hazardous material incidents, and address communications issues, such 

as radio interoperability, to ensure the timely transmission of critical safety information to 

responding officers. 

2. Ensure that police officers responding to hazardous material incidents are briefed on specific 

incident conditions, and are equipped with and trained on the proper use, capabilities, and 

limitations of appropriate protective equipment. 

3. Ensure that police officers receive hazardous materials – operations level training, and annual 

hazardous materials and air purifying respirator (APR) refresher training. 

4. Conduct exercises with the Glendale Fire Department to identify and resolve police/fire 

integration issues. Coordinate exercise planning with the Arizona Division of Emergency 

Management Exercise Officer and with the Maricopa County LEPC. Schedule periodic 

hazardous materials incident drills to ensure safe and effective responses to future hazardous 

materials incidents. 

Maricopa County Department of Air Quality 

1. Revise DPC’s permitted operating conditions to specify a minimum scrubber caustic 

concentration of 8 percent or more, as determined by laboratory measurement, with 

measurements taken daily and upon completion of each scrubber batch. 
DPC Glendale, AZ February 2007 

43

The Chlorine Institute 

1. Clarify the chemistry involved in over-chlorination incidents so that “Chlorine Scrubbing 

Systems, Pamphlet 89,” and other pertinent publications: 

• Ensure that the recommended practices and safeguards prevent, mitigate, and control 

hazardous releases due to bleach decomposition. 

• Provide sufficient detail on the safety and environmental consequences of overchlorination to enable companies to provide emergency responders with information on 

the potential characteristics of over-chlorination events, and on the best means of 

mitigating the bleach decomposition reaction following a release. 
DPC Glendale, AZ February 2007 

44

By the 

U.S. Chemical Safety and Hazard Investigation Board 

Carolyn W. Merritt 

 Chair 

 John S. Bresland 

 Member 

 Gary L. Visscher 

 Member 

 William B. Wark 

 Member 

 William E. Wright 

 Member 

Date of Board Approval 
DPC Glendale, AZ February 2007 

45

7.0 References 

Adams, Luke, et. al., 1992. Hypochlorous Acid Decomposition in the pH 5-8 Region, Inorganic 

Chemistry, 31, 3534-3541. 

American Institute of Chemical Engineers (AIChE), 2006. Design Institute for Physical Properties, 

Project #801, correlation for chlorine vapor pressure 

Center for Chemical Process Safety (CCPS), 2004. Inherently Safer Chemical Processes, A Life Cycle 

Approach, AIChE 

CCPS, 2001. Guidelines for Revalidating Process Hazard Analyses, AIChE. 

CCPS, 2001. Layer of Protection Analysis - Simplified Process Risk Assessment, AIChE. 

CCPS, 1999. Guidelines for Process Safety in Batch Reaction Systems, AIChE. 

CCPS, 1996. Guidelines for Writing Operating and Maintenance Procedures, AIChE. 

CCPS, 1995. Guidelines for Process Safety Documentation, AIChE. 

CCPS, 1995. Guidelines for Safe Process Operation and Maintenance, AIChE. 

CCPS, 1994. Guidelines for Preventing Human Error in Process Safety, AIChE

CCPS, 1993. Guidelines for Auditing Process Safety Management Systems, AIChE. 

CRC Press, 1980. Handbook of Chemistry and Physics, 61st ed. 

The Chlorine Institute (CI), 2006. Chlorine Scrubber Systems, Pamphlet 89, Edition 3, June 2006. 

CI, 2006. Sodium Hypochlorite Manual, Pamphlet 96, Edition 3, April 2006 

CI, 2004. Chlorine Institute Scrubber Survey, April 17, 2004. 

CI, 2003. Chlorine Customer Generic Safety and Security Checklist, Edition 1(R), November 2003. 

CI, 2001. Recommendations to Chlor-Alkali Manufacturing Facilities for the Prevention of Chlorine 

Releases, Pamphlet 86, Edition 4, April 2001. 

CI, 2000. Sodium Hypochlorite Manual, Pamphlet 96, Edition 2, May 2000. 

CI, 1998. Chlorine Scrubber Systems, Pamphlet 89, Edition 2, December 1998. 

CI, 1998. Estimating the Area Affected by a Chlorine Release, Pamphlet 79, Edition 3, April 1998. 

CI, 1997. The Chlorine Manual, Pamphlet 1, Edition 6, January 1997. 
DPC Glendale, AZ February 2007 

46

City of Glendale, Arizona Fire Department, After Action Report–Chlorine Release–DPC Industries, Inc., 

Incident Number GFD #2500353, GPD DR#03-137821. 

Department of Homeland Security. Homeland Security Exercise and Evaluation Program (HSEEP), 

http://hseep.dhs.gov (accessed January 2007). 

U.S. Environmental Protection Agency (EPA), 2000a. General Guide for Risk Management Programs

(40 CFR Part 68), RMP Series, EPA-550-B-00-008, May 2000. 

EPA, 2000a. Risk Management Guidance to Chemical Distributors (40 CFR Part 68), RMP Series, EPA 

550-B-00-005, May 2000. 

Kirk-Othmer, 1993. Encyclopedia of Chemical Technology, 4th ed. Vol.5. John Wiley & Sons, New 

York, 1993. 

National Response Team (NRT), 2001. 2001 Hazardous Materials Emergency Planning Guide NRT-1. 

NRT, 1990. Developing a Hazardous Materials Exercise Program: A Handbook for State and Local 

Officials NRT-2. http://ntl.bts.gov/DOCS/254.html (accessed January 2007).

Occupational Safety and Health Administration (OSHA). “Occupational Safety and Health Guideline for

Chlorine,” http://www.osha.gov/sltc/healthguidelines/chlorine/recognition.html (accessed June 2006). 

Powell Fabrication & Manufacturing Inc., ORP Electrode (product literature), printed by Solutions at 

Work. 
DPC Glendale, AZ February 2007 

47

Appendix A Chlorine Physical Properties and Exposure Limits 

Table 1A summarizes important physical properties of chlorine. 

Property Value / Units 

1 Molecular Weight1 70.9 

2 Vapor Specific Gravity (Air = 1.0)1 2.45 

3 Normal Boiling Point 

Temperature1 -29.2o

F (-34oC) 

4 Vapor Pressure at 32oF (0oC)2 38.9 psig 

5 Vapor Pressure at 77oF (25oC)2 98.5 psig 

6 Water solubility at atmospheric 

pressure and 77o

F (25oC)3 6.4 grams/liter (slightly soluble) 

7 Odor Pungent / Penetrating3

Table 1A. Physical properties of chlorine 

Sources: 

1. CRC Press, 1980. Handbook of Chemistry and Physics, 61st ed, p. B-93. 

2. AIChE, 2006. Design Institute for Physical Properties, Project #801, correlation for chlorine vapor 

pressure 

3. The Chlorine Institute, 1997. The Chlorine Manual, Pamphlet 1, Edition 6, January 1997, p. 48. 

Table 2A summarizes exposure limits for chlorine. 

Standard Setting 

Body 

Permissible Exposure, 

ppm Description 

NIOSH 0.5 Recommended Exposure Limit (REL) 

OSHA 1.0 Permissible Exposure Limit (PEL) 

NIOSH 1.0 Short Term Exposure Limit (STEL) 

AIHA 3 Emergency Response Planning Guideline Level 2 

NIOSH 10 Immediately Dangerous to Life or Health (IDLH) 

Table 2A. Exposure limits for chlorine 
DPC Glendale, AZ February 2007 

48

 

The PEL (OSHA) and REL (NIOSH) are time-weighted exposure levels for routine worker exposure. 

The STEL (NIOSH) and the PEL are ceiling (maximum) exposure limits. 

The Emergency Response Planning Guideline (ERPG) level 2 concentration is used to determine the toxic 

endpoint distance for estimating off-site consequences in the EPA’s RMP program. EPRG concentrations 

are issued by the American Industrial Hygiene Association (AIHA). 

Exposure to chlorine at concentrations at or above the IDLH (NIOSH) may make escape from a vapor 

cloud difficult due to severe eye and respiratory irritation. Serious health effects, including permanent 

harm, may also occur. Air purifying respirators (APRs), the type of respiratory PPE issued to the 

Glendale Police Department, may not be used in atmospheres containing chlorine concentrations above 

the IDLH.1

1

 APRs use absorbent cartridges to remove contaminants from breathing air. Saturation of the absorbent cartridges 

or leakage into the respirator’s face mask may expose personnel to toxic concentrations of contaminants. 
DPC Glendale, AZ February 2007 

49

Appendix B Bleach Over-chlorination Chemistry 

Based on an extensive literature search and discussions with industry and academic experts, the CSB has 

determined that the bleach in the DPC–Glendale scrubber likely decomposed following depletion (overchlorination) of caustic soda (NaOH), releasing chlorine and possibly other toxic materials into the 

atmosphere. The CSB’s research revealed a need for better guidance on the magnitude and duration of 

toxic releases that can occur in over-chlorination incidents. Such data will enable companies to properly 

size mitigation equipment and provide useful information to emergency responders. 

Chlorine is commonly scrubbed with caustic soda (NaOH) solutions. The chlorine moves from the gas to 

the liquid phase and reacts with the caustic soda to form sodium hypochlorite (NaOCl–bleach) and 

sodium chloride (NaCl - common salt), in accordance with: 

Reaction 1 Cl2 + 2 NaOH  NaOCl + NaCl + H2O (bleach formation reaction) 

Each pound of chlorine reacts with 1.13 pounds of NaOH, removing the NaOH from the circulating 

scrubber solution. Reaction 1 ceases when the NaOH is fully consumed. Any additional chlorine fed to 

the scrubber will not be captured and will be emitted to the atmosphere. 

Depletion of NaOH also leads to the rapid decomposition of bleach to form sodium chlorate and salt 

(NaCl), in accordance with reaction 2 (Adams et. al., 1992). Commercial bleach solutions typically 

contain between 0.2-0.5 percent caustic to maintain stability, with the exact amount of caustic determined 

by customer specifications. 

Reaction 2 3 NaOCl  NaClO3 + 2 NaCl (overall decomposition reaction) 

Reaction 2 is rapid at neutral pH and ambient temperature. The rate strongly depends on bleach 

concentration, implying an initially high rate of decomposition that slows sharply as the bleach 

concentration decreases. 
DPC Glendale, AZ February 2007 

50

The decomposition reaction proceeds through several steps in which significant amounts of hypochlorous 

acid (HOCl); chlorine monoxide (Cl2O); and hydrochloric acid (HCl) occur as intermediate species; that 

is, these materials are both formed and consumed in the overall reaction. Hypochlorous acid and chlorine 

monoxide are toxic and may contribute to hazardous emissions during over-chlorination incidents. 

Reaction 2 is exothermic (heat-producing) and increases the temperature of the over-chlorinated scrubber, 

further accelerating the decomposition rate. Decomposition eventually slows as the bleach is consumed, 

and may continue at a low rate for an extended period. 

Most of the hydrochloric acid intermediate formed is expected to be consumed converting bleach to 

hypochlorous acid, in accordance with: 

 Reaction 3 HCl + NaOCl HOCl + NaCl (intermediate step in decomposition 

reaction) 

The hypochlorous acid formed then decomposes to stable chlorate plus additional hydrochloric acid in 

accordance with: 

Reaction 4 2 HOCl + NaOCl  NaClO3 + HCl (hypochlorous acid decomposition 

reaction) 

The hydrochloric acid formed in this reaction is then available to convert additional bleach to 

hypochlorous acid, continuing the decomposition of the bleach to sodium chlorate. However, the reduced 

pH caused by hydrochloric acid formation and accumulation creates the potential for chlorine gas 

formation through: 

 Reaction 5 HOCl + HCl  Cl2 ↑ + H2O (chlorine gas formation) 
DPC Glendale, AZ February 2007 

51

 

The rate at which the pH changes during decomposition is not well documented, but is expected to remain 

at or above pH ≈ 2.0,1

 low enough to readily produce chlorine by Reaction 5. The chemical literature 

reviewed by the CSB does not address the amount of chlorine formed by this reaction in over-chlorination 

incidents. The CSB investigators estimate that as much as 1,920 pounds of chlorine may have been 

released at Glendale by this mechanism. 

A parallel bleach decomposition reaction described in The Chlorine Institute’s “Sodium Hypochlorite 

Manual Pamphlet 96” is: 

 Reaction 6 2 NaOCl  O2 ↑ + 2 NaCl 

Reaction 6 produces salt and gaseous oxygen as products. This decomposition reaction is enhanced at 

high temperatures and low pH, the conditions created in an over-chlorinated scrubber by reaction 2. It is 

thus possible that reaction 6 contributes to the decomposition of bleach in over-chlorination incidents. 

While the products of reaction 6 are not hazardous, the oxygen produced could act as a stripping gas, 

enhancing the emission of volatile toxic materials from the scrubber. 

Raising the pH by adding adequate excess caustic to the solution is expected to interrupt both reactions 2 

and 6, and to absorb any chlorine gas dissolved in the scrubber solution. Once an over-chlorination 

incident has begun, this is the only way to interrupt the decomposition reactions and stop toxic gas 

emissions. 

Over-chlorination incidents, including the incident at DPC-Glendale, are described as causing 

considerable rumbling and shaking of the equipment involved. It is likely that this results from the 

1

 This is based on expert testimony and the stability of the chlorate salt formed in the decomposition reaction. No 

data were found documenting the changes in pH during over-chlorination incidents. 
DPC Glendale, AZ February 2007 

52

formation of gaseous products and the rapid heating of the scrubber contents by decomposition reactions 

2 and 6. 

While the chemical pathways in bleach decomposition appear to be well understood, published data do 

not address the identities and quantities of the toxic materials emitted from over-chlorinated scrubbers, 

nor are changes in system temperature and pH described in detail. This information is needed by 

companies to size mitigation equipment and to provide accurate information to emergency responders 

during over-chlorination incidents. 
DPC Glendale, AZ February 2007 

Appendix C DPC – Glendale Incident Time Line 

11/17/2003 - 6:00am

Shift starts

11/17/2003 - 7:00am

Bleach Production Report

Caustic 1.60%

ORP 490 mV

11/14/2003 - 2:00pm

Bleach Production Report

Excess Caustic 1.76%

ORP 480 mV

11/17

Begin 

11/17/2003 - 11:15am

Second ORP Safety Meter 

Alarm

Acknowledged, chlorine 

continues to flow, operator 

prepares to take sample

11/17/2003 - 10:15am

First ORP Safety Meter 

Alarm 

Acknowledged, chlorine 

continues to flow

11/17/2003 - 10:00am

Bleach Production Report 

Caustic 1.18%

ORP 510 mV

11/17/2003 - 11:30am

Chlorine release. 

Emergency shutdown & site 

evacuation

11/1

91

chlo

neighb

11/17/2003 - 11:35am

First alarm - hazardous materials 

incident

18 emergency responder units 

dispatched (PD, FD)

11/17/2003 - 11:38

Initial responders on site 

(Phoenix Fire Dept) 

11/17/2003 - 15:49

DEQ below detection 

limit readings except 

at 600 ft perimeter

11/17/2003 - 20:54

Incident Closed by 

Glendale FD

11/17/2003 - 13:17

Evacuation message delivered to 

4128 phones in the evacuation 

area in English, Spanish and 

TDD

11/17/2003 - 14:12 

DEQ Hazardous Air Response 

Team takes first sample at West 

Pasadena - reading less than 

0.2 ppm

11/1

 Chlorine 

ft radius, 1

25-35+ 

Respond

11/17/2003 - 17:30

DPC adds caustic to 

scrubber. DEQ reports 

brief emission. 

Scrubber stabilized.

53
DPC Glendale, AZ February 2007 

Appendix D DPC – Glendale Incident Event Tree

54
DPC Glendale, AZ February 2007 

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