- Introduction and Scenario
- 1.1 Chrome Plating
- 1.2 Scenario
- Risk Assessment
- 2.1 Potential Exposure Routes
- 2.2 Health Effects
- 2.2.1 Chrome Holes
- 2.2.2 Damaged Septa
- 2.2.3 Other Health Effects
- 2.3 On Site Exposure
- Monitoring of Exposure
- 3.1 Sampling
- 3.2 Sampling Methods
- Prevention of Worker Ill Health
- 4.1 Prevention Strategy
- 4.2 Engineering Controls
- 4.3 Work Practices
- 4.4 Monitoring
- Conclusion
- Bibliography
1.1 Chrome Plating
In electroplating, an electrochemical reaction causes a metal coating to be applied to a metal part. Electroplating with hexavalent chromium (hard chrome) is conducted to increase a parts surface hardness, wear resistance, and corrosion resistance.3 During the chrome plating process, a current is passed through the chromic acid solution from a negative electrode (anode) to the part that acts as the positive electrode (cathode). Chrome plating baths are maintained at a temperature of approximately 180 degrees Fahrenheit.4
Exposure to the chemicals and emissions associated with chromium plating present serious worker health concerns. In 1994, EPA estimated approximately 90,000 workers were employed in chromium plating facilities in the United States.3 A major portion of these chrome plating facilities are small businesses, lacking both the detailed industrial hygiene knowledge of chromium's health effects and the resources to implement strong industrial hygiene programs.4
1.2 Scenario
Smith Hard Chrome Plating Incorporated is a mid sized (100 employees) hexavalent chromium plating facility located on the North Carolina piedmont. The expanding company recently implemented a third shift to undertake a large contract plating operation for an automobile company. Under the terms of the contract, Smith Hard Chrome will use a new barrel plating method to coat the small exterior chassis components. Barrel plating operations involve placing a large number of parts in a barrel. The entire barrel is immersed manually into the plating solution. Sieve-like walls allow the plating solution to enter and come in contact with the parts. Rotation of the barrel permits free movement of the parts and uniform plating coverage on the part surface.
In the last month, 3 of the 10 workers on the shift have displayed symptoms consistent with exposure to elevated levels of chromic acid. Two workers have developed ulcers (chrome holes) on the backs of their hands and the other worker is experiencing frequent nose bleeds. Push-pull ventilation systems are installed on the plating baths but no covers are used. A push-pull system provides a horizontal curtain of air over the surface of the tank and aids in the capture of emissions and the escape of emissions above the curtain to the surrounding air.4
The company has always demonstrated a concern for the health of their workers and adopted a proactive approach to industrial hygiene. The health concerns were raised by the workers during their monthly medical examinations.
In an effort to address these health concerns quickly, the company has called in an industrial hygiene consultant to assess the cause of the health problems. This paper serves as the report.
2.1 Potential Exposure Routes
Chrome plating operations are regarded as one of the most hazardous of plating operations.5 Chromium plating baths consist of an acidic solution and the primary substance of concern is chromic acid mist which may be inhaled by workers or come in contact with the skin. During the plating process bubbles of hydrogen and oxygen form in the vicinity of the part. As the hydrogen and oxygen bubbles emerge from the solution, they entrain plating solution droplets, carrying them as a mist into the air above the tank. The lower the efficiency of the plating operation, the greater the production of hydrogen and oxygen, and the greater amounts of acid mist. Hard chromium plating efficiencies are extremely low, typically 12-15% which means large quantities of misting are common with these operations.4
2.2 Health Effects
Chromic acid is an oxidizing agent that is irritating to all exposed tissues. Industrial hygiene studies have consistently found problems with skin lesions, nasal ulceration and perforation and other nasal mucosa problems (Table 1).
2.2.1 Chrome Holes
Industrial hygiene studies have frequently reported "chrome holes" on workers in the chrome plating industry. The term "holes" was given to the ulcers because they are penetrating. These skin lesions caused by the plating solution's caustic nature heal slowly but are not usually painful. Infection is often a problem. The most common sites are around the fingernails, surfaces of exposed finger joints or eyelids and sometimes on the forearms.4
Study Chromic acid Health effects
concentration
(mg/m3)
Bloomfield and 0.06 - 1.8 Ulcerated and perforated septa,
Blum (1928) inflamed mucosa, nosebleed, and
respiratory tract irritation.
Kleifield and 0.09-0.73 Perforated and ulcerated septa.
Rosso (1955) 0.12-0.57
Mitchell (1969)
Markel and Lucas 0.0005-0.003 Chrome ulcers
(1973)
Cohen and 0.0029 Chrome ulcers and septum damage.
Kramowski (1973)
Royle (1975) NA High rates of lung and pleural
cancer, gastrointestinal cancer, and
other types of cancer.
2.2.2 Damaged Septa
Inhalation of the acid mist causes irritation of the nasal mucosa, inflammation, purulent discharge, formation of crusts, and some difficulty breathing. Painless and chronic ulcers may form in the cartilaginous portion of the septum. This process ends in perforation of the septum often causing discomfort and making mouth breathing necessary.2
2.2.3 Other Health Effects
Today, chromic acid is considered a carcinogen by the National Institute of Occupational Safety and Health (NIOSH). One study concluded the latency period for chromium induced lung cancer is greater than 20 years; exposure duration may be as low as two years.5
The current NIOSH REL (10 hour TWA) is 0.001 mg/ cubic meter and the OSHA PEL is 0.1 mg/ cubic meter (Ceiling) for chromic acid (Table 2).6
Potential Exposure Symptoms
Routes
Inhalation Respiratory system Liver and kidney
Contact irritation damage Leukocytosis
Ingestion Nasal septum or leukopenia
perforation Monocytosis
Skin ulcers Eosinophilia
Dermatitis
Carcinogenic
Aside from breathing or indirect exposure to chromium, the health effects described above can be caused by direct contact with the skin or nose. Thus poor work practices and personal hygiene can cause health effects even when air levels are low. Thus if ambient air levels of chromium are below TLV levels health problems may still be prevalent due to poor work practices and personal hygiene.
2.3 On Site Exposure
Barrel plating operations in which the barrel consistently rotates, results in greater agitation of the bath and potentially greater production of acid mist. Additionally, the process is more labor intensive and increased manhandling presents a greater potential for skin exposure. The worker's major tasks include loading and unloading parts from the barrels by hand, mixing and maintaining solution levels, and cleaning of plating tanks.4
Initial monitoring through the use of personal sampling revealed periodic concentrations of chromic acid exceeding the TLV. Thus a more extensive monitoring program was undertaken.
3.1 sampling
Personal, plating bath, and area samples should be collected.
Personal sampling - Attached to the front side of the shirt so as to place them in the breathing zone. These samples give a direct indication of the worker exposure and the existing controls effectiveness.
Plating bath - Place one monitor directly over the surface of the tank (if possible) to sample the air before it is effected by ventilation. Also, assess air movement over and around the tanks. Both methods will demonstrate the effectiveness of the existing controls.
Area samples - Placed at fixed locations around the plating tanks and the building.
3.2 Sampling Methods
Long term sampling provides a more accurate indication of worker exposure as concentrations may vary throughout the day depending on the workers job duties e.g. exposure may be higher on cleaning days. Also, individual sampling data can be reviewed to determine if an individual worker's personal hygiene or work practices are related to high exposure levels.
Chromic acid - Use colorimetric tubes for instant evaluation and silica gel absorbent tubes and laboratory analysis to determine long term exposure levels.
Assessment of ventilation system effectiveness - Periodically determine the ventilation exhaust rate by measuring air velocities around the tank. The ventilation exhaust rate is the ratio of the volumetric flow rate being drawn into the local exhaust ventilation system to the surface area of the tank. High exhaust ratios reduce airborne concentrations and also reduce unwanted air currents moving across the surface of the tank. In addition frequently assess air movement around the plating tanks with smoke tubes to qualitatively evaluate exhaust effectiveness.4
Monitoring revealed continued high chromic acid exposure levels in the vicinity of the plating tanks. Ventilation exhaust rates were measured at 180 ft3/ ft2 . Qualitative air movement assessment with smoke tubes revealed air moving form the tanks to the surrounding line areas. This indicated the shortcomings of the push pull ventilation system.
4.1 Prevention Strategy
In an effort to reduce chromic acid exposure to levels below the TLV, occupational exposure should be controlled by the application of process engineering measures and improved work practices. Furthermore, a more extensive monitoring program should be implemented to verify the continued effectiveness of these measures.
4.2 Engineering Controls
The following engineering controls should be designed into or incorporated around the production processes to minimize worker exposure.
Tank Ventilation - The company should upgrade the push pull ventilation system to maintain ventilation exhaust rates of 250 ft33/min/ft2. Exhaust rates at this level are recommended in the Industrial Ventilation Manual.4
General Ventilation - Strong drafts from open windows and doors blowing across the surface of a tanks can reduce the effectiveness of exhaust systems. Also, do not use standup portable fans to moderate weather heat since this will disrupt local ventilation.
In addition, consider constructing baffles around plating tanks to suppress interfering air currents.4
Tank Covers - The single most important parameter affecting average tank concentration is whether or not a tank is covered. Covers generally consist of plastic chips, beads, balls, or foam blankets which float on the surface of the tank reducing releases of the chromic acid mist to the ambient air.
Even if an elaborate ventilation system is in place a plating tank may still be unable to decrease emissions below the TLV if left uncovered. Up to 20 fold drops in chromic acid concentrations have been recorded after the installation of a cover. A full cover contains the mist in the region immediately above the tank surface and shields the tank from room air currents.4
Surfactants - Another method for reducing emissions involves the use of surfactants. These chemicals serve to lower the bath surface tension, permitting the release of smaller bubbles which reduces the release of chromic acid to the air.4
Plating Operation Efficiency - The generation of acid mist from the bath is directly proportional to the efficiency of the plating operations i.e. the more efficient the operation, the lower the mist emission rates. Ensuring that all bath operating parameters such as temperature and chemical concentrations are set for maximum efficiency will reduce acid mist production. Regular cleaning of equipment such as electrical contacts will also help maintain optimum efficiency.3
4.3 Work Practices
The following additional suggestions and work practices should be used to minimize health and safety hazards to workers and their families.
Training
- Make employees aware of the potential safety and health hazards and the need for PPE and other protective measures.
- Provide bonuses for outstanding performance including those in the health and safety field.
Personal Protective Equipment
- Provide fit tested respirators for use during routine maintenance such as cleaning tanks.
- Wear face shields while handling chemicals or filling tanks.
- Provide rubber gloves, boots, safety glasses, and an apron for a person working around chromic acid tanks.
Hygiene
- Prohibit eating, smoking or drinking on the plating floor.
- Construct a separate and separately ventilated lunchroom.
- Install a wash basin in the proximity of eating area.
- Routinely clean eating areas.
- Collect work clothes in large receptacle for laundering by company. This will prevent continued worker exposure after hours and exposure to other members of workers family.
Work Practices
- Ensure proper part rinsing over plating tank to minimize dragout of chromic acid and minimize opportunity for later worker contact during part handling.
- Investigate process automation to reduce worker exposure.
General Safety
- Organize a safety committee consisting of management and worker representatives to conduct periodic inspections.
- Have local fire department tour facility to assess and comment on fire hazards, assist with evacuation plans, and to familiarize themselves with the locations of the tanks and other potential hazards in an emergency.
4.4 Monitoring
Monitoring of worker exposure to chromic acid should be continued regularly in the future. Each month workers should be fitted with monitors and exposures evaluated. Continued monitoring of the ventilation system should also be conducted (perhaps bi-monthly) using smoke tubes and area samples.
All employees should receive periodic (quarterly) medical examinations. During these examinations, nasal passages should be inspected regularly and careful attention should be given to trivial injuries of the skin since they may lead to serious chrome ulcers.
The cause of the high chromic acid levels was thought to be due to a combination of factors. The primary cause was the increased agitation caused by the barrel plating operations and the fact that covers were not used to prevent acid releases from the plating baths. In addition, the ventilation system was not operating at an effective exhaust rate. Furthermore, excessive drafts and wind currents caused by open doors and windows inhibited the efficiency of the exhaust system. Installation of the suggestions above should prevent high exposure levels in the future.