Determining safe concentrations of dust can be a complicated process; some dust particles are too small to see. Government, academic and industry scientists have developed guidelines for dust concentrations based on predicted effects. Obviously, guidelines for products that contain highly poisonous chemicals are set much more strictly than for products that may simply be irritating.

Guidelines for occupational exposure to borate dusts were adopted by the California Occupational Safety and Health Administration (Cal-OSHA) in the early 1980s. At that time, following the recommendation of ACGIH¹ they believed that two products - boric oxide and Neobor® (sodium tetraborate pentahydrate) - were more irritating than borax decahydrate. Permissible exposure limits (PELs) were set at one milligram per cubic meter of air for the former and five milligrams per cubic meter for the latter. Following review of a study showing the equivalency of all three products², Cal-OSHA raised their PEL in the early 1990s to five milligrams per cubic meter.

Studies in the late 1980s and early 1990s evaluated workers at Boron Operations through questionnaire and real-time exposure measurements. A report of this investigation¹ concluded that there were associations between levels of borate exposure and symptoms of acute respiratory irritation. However, experts found no evidence of long-term or chronic effects on lung function over a seven-year interval, nor did they find differences among the three different borate products. They concluded that "the totality of the evidence suggests that the current levels of sodium borate exposures in this plant are generally associated with no more than moderate irritant responses." One limitation of this study was that the symptoms and the severity of symptoms were not measured by any standard scale or criteria - what one person reported as "noticeable" another may have classified as more irritating.

To objectively address this limitation, Dr. William Cain of the University of California, San Diego, has developed a method to quantify dust irritation in a way that meets the highest international standards for occupational health and industrial hygiene. With support from Borax, Dr. Cain and his colleagues have investigated several dusts, including calcium sulfate (a dust that may cause no or minimal irritation), calcium oxide (which does cause moderate to severe irritation), and sodium borate.

Subjects rode an exercise bicycle for 20 minutes while exposed to dust particles². During and after the exposure they were asked to judge how they felt in comparison with the irritation caused by specific concentrations of carbon dioxide. This normalization step allowed better comparison of how different people rate their response to the dust concentrations. In addition to the sensory reactions, breathing and heart rates, blood oxygen levels, ease of breathing and nasal secretions were measured.

The results of this study are being published³. As expected, the calcium oxide was felt to be more irritating than the calcium sulfate. However, the sodium borate was felt to be less irritating than the calcium oxide when exposures were compared on a mass basis (milligrams of dust per cubic meter of air). The concentrations of dissolved material in the nasal mucus provided a clue: when concentrations were expressed as the number of molecules present, the level of intensity felt by the subjects was the same. This research suggested that the potency of dusts like sodium borate and calcium oxide was related to the number of molecules present - at equal molecular concentrations in the air or dissolved in mucus, these materials have equal effects in the nose.

A molecule of sodium borate (molecular weight 291) is about five times the mass of a molecule of calcium oxide (molecular weight 56). So, it takes about five times more sodium borate by weight to match the molecular concentration of calcium oxide. Calcium oxide is a dust that is widely recognized as irritating and occupational exposure guidelines or limits have been set for it at two (ACGIH) to five (OSHA) milligrams per cubic meter of air. If the five-fold factor is applied to set an equivalent guideline or limit for sodium borate, then a strong case can be made that an occupational exposure to 10 milligrams per cubic meter of air or less of sodium borate dust will protect against unacceptable irritation.

Dr. Cain is currently investigating the responses to boric acid using this same procedure. These preliminary findings confirm that our existing practices and internal standards are adequate to protect our employees from unsafe exposures to boron dusts. Borax will continue to support environmental and behavioral safety systems that enhance the safety of our workforce, including research that may challenge our understanding of safe occupational practices.