Manganese (Mn) is element number 25 on the periodic table, is CAS number 7439-96-5 and has an atomic weight of 54.9. It’s a naturally-occurring mineral and found in all three major types of (igneous, sedimentary, and metamorphic) of rock. One of its unique forms is nodules deposited on the surface of the ocean floor. It is the 12th most abundant element in the earth. Chemical forms of manganese include oxides, sulfides, carbonates and silicates are found: the prevalent form is manganese dioxide.
It’s an essential mineral in the human body and responsible for a number of functions including metabolism of glucose and amino acids, bone development and antioxidant enzymes. It also has a role in blood clotting and reduction of inflammation. In fact, it is involved with over 300 metabolic reactions. Excess uptake via food, water or inhalation can be toxic to the nervous system. Calcium, magnesium and iron are elements in the diet that may influence the uptake and metabolism of magnesium. Inadequate dietary manganese has been associated with metabolic disordered, osteoporosis and cardiovascular disease. There is no recommended daily intake, but 2.3 mg (male) and 1.8 mg (female) is considered acceptable. It been suggested that approximately half of the US population may have insufficient manganese in their diet.
Manganese can exist in 11 oxidation states from -3 to +7. The +2 valence is found in most biological systems and +7 valence in permanganate (a powerful oxidant). Methyl cyclopentadienyl manganese tricarbonyl is added to gasoline as an antiknock agent and another form (manganese ethylenebisdithiocarbamate) is used as a fungicide. Toxicity occurs in humans with inhalation of respirable (particles less than 5 microns in diameter) manganese dioxide.
Manganese health effects for workers
Prolonged exposure to high levels of manganese can results in a condition known as Manganism, which is a characterized by asthenia, insomnia, mental confusion, metal fume fever, chest tightness, and breathing difficulty. Other symptoms may include low-back pain, vomiting, malaise, weakness, exhaustion and kidney damage.
Within the past decade it’s been recognized that exposure below occupational exposure limits can affect the nervous system and result in a Parkinson-like condition. According to the National Institute for Occupational Safety and Health (NIOSH) “Recent studies indicate neurological and neurobehavioral deficits may occur when workers are exposed to low levels of manganese (less than 0.2 milligrams per cubic meter ( mg/m3) of air) in welding fumes. These effects include changes in mood and short-term memory, altered reaction time, and reduced hand-eye coordination.”1
Sources of Manganese in Welding
For this article the term “cutting” applies to arc cutting/gouging or thermal cutting such as an acetylene/oxygen cutting. Manganese improves the mechanical properties (tensile strength and hardness) of steel. Typically mild steel is generally considered a low hazard with respect to metals, although it typically contains manganese. It is contained in steel alloys and is also used as a flux in shielded metal arc and flux core arc welding and submerged arc welding. The safety data sheet should be checked for the base metal, filler metal and any fluxes.
Occupational Exposure Limits (OEL)
The hazardous metals that first come to mind related to welding and cutting are lead, chromium, zinc and perhaps beryllium. Manganese doesn’t have the same recognition in terms of risk in the general population.
OSHA - Federal OSHA has a permissible exposure limit (PEL) of 5 milligrams per cubic meter (mg/m3) of air. This is a ceiling or instantaneous value that should never be exceeded. The standard has been in effect since the original OSHAct and was based on the 1969 Threshold Limit Values (TLV) from the American Conference of Governmental Industrial Hygienist (ACGIH). This value is consider outdates and not protective, not just because of its age, but also based on other OEL and findings related to the health effects.
ACGIH - The ACGIH has two TLVs for manganese, both of which are time-weighted averages (TWA) unlike OSHA’s ceiling value. The TLV for respirable manganese is 0.02 mg/m3 and the TLV for inhalable standard is 0.1 mg/m3. The respirable fraction of airborne particulates is those that penetrate the deepest parts of the lung or what’s known as the gas-exchange region where there are fewer defense. Inhalable particulate matter is hazardous when deposited in any portion of the lung and have an aerodynamic diameter less than 100 microns.
NIOSH – The NIOSH standards are different from both ACGIH and OSHA. Their standards are known as recommended exposure limits (REL). NIOSH’s TWA for manganese is 1 mg/m3. In addition, they have a 15-minute, short-term exposure limit (STEL) for this substance, which is 3 mg/m3. According to NIOSH the level at which manganese becomes immediately danger to life and health (IDLH) is 500 mg/m3.
Control of worker exposure to manganese associated with welding and cutting are the same as other metals. The best approach is elimination of this metal from the base metal, flux or filler wire/rod. Other options include change from manual to automated welding/cutting, local exhaust ventilation and administrative controls. Personal protective equipment in the form of respirators is generally seen as a last choice, but can be effective if properly used. Depending the job, respirators may be necessary. A half-mask with air-purifying respirator (APR) for particulates (example N-100) provides a limited level of protection. A full-face, APR provides better protection. In extreme cases where high levels of airborne manganese are anticipated an airline respirator may be necessary.
Manganese air sampling and analysis
Welding and cutting typically produces a significant number of small particles in the respirable range. Air monitoring should be conducted for both respirable and inhalable manganese, although higher readings relative to the OEL are expected with the respirable fraction. Recognized sampling protocols (and OSHA) call for air sampling under the welding helmet. This is not feasible for respirable samples of manganese using a cyclone, but can be done with a parallel particle impactor (PPI) for the respirable fraction. The IOM sampler can be used for both respirable and inhalable or a button aerosol sampler can be used for the inhalable fraction. Pump flow rate will depend on the device being used to collect the sample.
The analysis should be performed by an accredited lab. The Industrial Hygiene Laboratory Accreditation Program (IHLAP) is designed specifically for laboratories involved in analyzing samples to evaluate workplace exposure. Participation assists the laboratory in maintaining high quality standards. The IHLAP has been accrediting labs for more than 40 years.
Inductively coupled plasma with atomic emission spectroscopy (ICP-AES) NIOSH 7303 is one method used to analyze the air samples for manganese.
For more information: https://www.cdc.gov/niosh/topics/welding/default.html