Introduction
The field of occupational hygiene grew out of the need to protect workers from physical, chemical, biological and ergonomic hazards at the workplace. The International Labor Organization (ILO) estimates that 440,000 workers worldwide die each year from exposure to hazardous substances (Zaracostas 2005, p.656). The major goals of occupational hygiene encompass anticipation, detection, assessment, and control of a hazard (Vincent 2005, p. 649).
At the workplace, the most common hazards originate from the use and handling of chemicals and chemical products (Steve & Rampal 1999, p.79). This paper gives an overview of the occupational aspects of chemical hazards and exposure. The first part discusses the nature of chemical hazards at the workplace. Next, the regulatory measures adopted in Australia are examined. The details of the limits of these regulations are explored followed by practical aspects of sampling, quality control, control measures and the interpretation of sampling results. A specific case on respirable crystalline silica is also presented.
Chemical Hazards
Chemicals represent the most significant hazard to health in a workplace setting. The major forms of chemical forms at the workplace are dust (e.g. silica, coal, lead and asbestos), mist (e.g. acid mists, chrome plating etc), gases (e.g. Chlorine, Sulphur dioxide, Ozone), fumes (smoke metal fumes) and vapours (e.g. chlorinated solvents, amines, alcohols etc). Entry into the body may occur by inhalation, absorption through the skin or rarely, by ingestion/swallowing. The effects of chemical exposure range from acute to chronic. These effects may be exhibited by corrosion of tissue such as skin or lung, irritation of the skin or lung tissue, asphyxiation among others. Long term effects may include cancers due to gene mutations, liver failure, brain damage, reduced birth weight, miscarriage, Asthma etc (Creely et a. 2005, p. 106).
Applicable Control Legislation
In Australia, the regulatory frameworks for occupational hygiene regarding chemical hazards and exposure are contained in several legislations. The primary regulation is the National Model Regulations for the Control of Workplace Hazardous Substances [NOHSC: 1005 (1994) (EBSCO chemical Controls Watch 2010). This regulation is aimed at protecting workers from potentially harmful chemicals to health. It requires that employers provide employees engaged in a chemical-handling activity with Material Safety Data Sheet (MSDS).
The MSDS should provide details of the nature of the chemical(s) in use that have been categorized into three groups: Type I, Type II and Type III. Type I chemical category is made up of chemicals likely to cause adverse health effects ranging from carcinogens, mutagens, teratogens to various corrosives. Employers are also obligated to ensure correct labelling, registration and risk assessment of hazardous substances.
According to an EBSCO Chemical Controls Watch report (2010), a chemical inclusion in this group is based on exposure standards listed in the Adopted National Exposure Standards for Atmospheric Contaminants in the Occupational Environment [NOHSC:1003 (1995). Additionally, its listing warrants workplace exposure limits exceeding the “exposure limits” specified in another regulation by the NOHSC- Approved Criteria for Classifying Hazardous Substance [NOHSC:1008(2004). In addition to these broad regulations, there are also other control standards specially developed for some specific chemicals.
These are the National Standard for Control of Inorganic Lead at work, the National Code of Practice for the Safe Use of Vinyl Chloride and the National Standard for Synthetic Mineral Fibers. The national regulations for hazardous substances developed by the National Occupational Health Commission (NOHSC), now SafeWork Australia, only lays the necessary frameworks for the regulations of such substances. Territorial regions can set up their regulations in line with these frameworks. For example, Victoria regulates hazardous chemicals under its own enacted Occupational Health and Safety Regulations, 2007 (Victorian Trades Hall Council’s (VTHC) 2011).
Exposure Standards for Chemicals
The Australian Adopted National Exposure Standards for Atmospheric Contaminants in the Occupational Environment (Hereafter NOHSC:1003(1995) defines exposure standard as “an airborne concentration of a particular substance in the worker’s breathing zone, exposure to which, according to the current knowledge, should not cause adverse health effects nor course undue discomfort to nearly all workers” (p.70). These limits, based on dose-response studies are believed to be safe to workers even with repeated exposures (Sauleau et al. 2002, p.101). The limits of exposures applicable in Australian regulations are: “time-weighted average (TWA), peak limitation and short-term exposure (STEL)” (NOHSC:1003-1995, p.70).
TWA is interpreted as “the average airborne concentration of a particular substance when calculated over a normal eight-hour working day, for a five-day working week” while STEL is defined as “a 15 minutes TWA exposure which should not be exceeded at any time during a working day even if the eight-hour TWA average is within the TWA exposure standard” (p.70). Peak limitation is explained as the “maximum, or peak concentration of a particular substance determined over the shortest analytically practicable period which does not exceed 15 minutes”(p. 70). These limits are expressed in numerical values, namely: parts per million(ppm), milligrams per meter cubed (mg/m3) and fibres per millilitre (for fibre measurement by membrane filter method) (NOHSC:1003(1995), p.71). NOHSC:1003 (1995) lists TWA, STEL and peak limits for hundreds of workplace chemicals together with these exposure levels.
Assembling a complete list of exposure limits for chemicals is an involving process that involves a series of steps that involve hazard identification, risk assessment and risk evaluation (Oldershaw and Fairhurst 2001, p. 291). Significantly, acceptability and compliance mostly rely on a consensus reached on such limits by various stakeholders such as employers, trade unions health professionals and the government. In recent times there has been a cost-benefit approach to such processes whereby the benefits of standards are compared to the cost of implementation (Sandra and Rampal 1999, p. 118).
Sampling Methodology and Limitations
Sampling is an important step in the assessment of chemical exposure at the workplace. It mainly involves collecting samples of gases, vapour, dust, mists and fumes on the body of workers (mainly in the breathing zone) and the workplace air to determine the amount of contaminating agents. Samples gathered may provide immediate inference or may require further laboratory analysis depending on the sampling device used. The method employed may involve grab sampling or continuous sampling (Boss and Day 2001, p.118).
In grab sampling samples are gathered over short durations throughout an equally short period that may range from a few seconds to a minute (Brown and Monteith 1995, p. 371). This means this method is not suitable for contaminants with fluctuating concentration or in the determination of a time-weighted limit. On the other hand, continuous sampling may involve collecting samples over a much longer period and interval using either a single sample or multiple samples (Boss and Day 2001, p.118).
The type of sample required will determine the choice of method to be applied. In sample gases and vapours, direct-reading instruments have gained widespread applicability. These instruments include Detector tubes, Whole air samplers, Bubblers (liquid sorbents) and solid sorbent samplers
Both direct-reading methods and integrated methods may be applied in sampling procedures. In an integrated method, chemical samples of the workplace air are collected and later analysed in the laboratory. Direct-reading methods involve the use of Direct Reading Devices (DRDs) and are primarily used to monitor airborne pollutants in form of gases, vapours, and aerosols (BOHS 2001). Commercial DRDs operate on chemical and physical principles such as mass spectrophotometry, para-magnetism, chemiluminescence, catalysis, semi-conductivity, flame ionization, infrared, electrochemistry, etc. Modern direct-reading devices have further been enhanced by the integration of sound alarms and the ability to record data for peak, STEL and TWA. Their major limitation is that they are prone to interferences (chemical or physical) resulting in inaccuracies in measurements (BOHS 2001).
Sampling may also be active or passive. Inactive sampling, the air is drawn through a sampling medium such as a sorbent by use of a pump or vacuum. On the other hand, passive sampling is based on the diffusion of air particles from high to low region of concentration and do not require the usage of pumps. Both passive and active samplings possess several limitations. Active air sampling requires the use of expensive devices that require complex calibrations for the airflow rate (Bohlin, Jones and Strandberg 2007,).
Air-sampling pumps are also heavy, bulky prone to break down and require frequent calibration for accuracy (Harper 2004, p. 407). The limitations for passive sampling include higher recurring costs, post interference due to residues and the fact that most passive samplers lack a backup section (Harper 2004, p. 411). Another drawback is that most passive samplers have a fixed uptake rate and thus are limited in measuring extremes of concentrations (Harper 2004, p. 407). They have also suffered poor acceptance in the market partly influenced by a lack of endorsement by standard bodies and occupational health agencies (Harper 2004, p. 407).
Quality Control and Operator Safety
Quality control is an important aspect of occupational hazard assessment. Concerning chemical hazards, this will involve adhering to the basic requirement such as following the correct strategy and methods, documentation of all procedures and the use of representative samples.
Validation of sampling and test methods is important to ensure the accuracy of the result. Validation should provide for estimation, with acceptable accuracy, of extremes of concentration of the hazardous material.
Record keeping is central in the quality control of any laboratory. Field information such as temperature, sampling location and possible interfering compounds should be well stored (NIOSH Manual of Analytical Methods 2003). Field blanks, which are used to estimate pre-sampling and post-sampling contamination require a quality control strategy that should be formulated before the actual sampling.
Measurement procedures are perhaps the most important aspect of occupational hygiene quality control. There should be written guidelines for any measurement method. These procedures should also be evaluated periodically to confirm their performance. Standard solutions and other reagents should be of correct purity to avoid errors in the quantitation of results. All blanks (e.g., reagent, media and field blanks) should of correct measurement to minimize errors in analysing field samples. The use of blind samples should be such that they will offer the right degree of confidence in any independent analysis and not be any source of confusion or errors. Recovery studies are important in cases where there is a need for the separation of the contaminating agent from its media. They can be useful as analytical accuracy enhancing checks. Sampling instruments require calibration whenever a deviation is detected. Calibration is central to ensuring accurate results.
Some of the instruments that require calibration include bubble tubes and sampling pumps. For credible results, it is also important to select accredited laboratories. In the course of sampling, it is important to maintain communication with the project’s laboratory. All laboratory results should be thoroughly scrutinized. In case of any discovered flaws, a retest or resampling should be considered. The occupational hygienist should also observe his/her safety during sampling activity.
Broken glass from sorbent or calorimetric tubes should be handled with care. This also applies to toxic and inflammable substances such as impinge solutions. For confined spaces, any test or sample gathering should be conducted from the outside. Apart from adhering to control limits, employers are also required to undertake another measure to limit workers exposure to chemicals. They are required to provide workers handling chemicals with personal protective equipment. Their establishment structures are also required to have an efficient ventilation system for letting out contaminants. Other measures to be observed include maintenance of equipment, medical examination and testing of workers, training and supervision.
Interpretation of Monitoring Results
After laboratory analyses of the samples have been completed, the interpretation of the results can be made. The NOHSC requires that interpretation of exposure standards be undertaken by qualified and experienced personnel. In most cases based on the mandatory Australian regulation laws, compliance can be deduced by comparing the results with permissible Exposure Limits contained in the Adopted National Exposure Standards for Atmospheric Contaminants in the Occupational Environment [NOHSC:1003 (1995).
However, as a policy, the National Occupational Health and Safety Commission requires the use of the Guidance Note on the interpretation of Exposure Standards for Atmospheric Contaminants in the Occupational Environment in interpreting exposure limits contained in the National Exposure Standards for Atmospheric Contaminants in the Occupational Environment (Australia. National Occupational Health and Safety Commission 1995). The current publication is the Guidance Note on the Interpretation of Exposure Standards for Atmospheric Contaminants in the Occupational Environment NOHSC 3008(1995) 3rd Edition. This document puts it clearly under its policy statement that “exposure standards are guides to be used in the control of occupational health hazards [and] should not be used as fine dividing lines between safe and dangerous concentrations of chemicals”.
Case study: Respirable Crystalline Silica
Crystalline silica is a type of silicon dioxide whose main forms are quartz, cristobalite and tridymite. The occupational hazardous silica, respirable crystalline silica (RSC) refers to very fine crystalline silica dust less than 10µm in diameter capable of infiltrating into the non-ciliated airways of the lungs through inhalation (AIOH 2009, p.7). RSC is known to occur in workplaces such as mining, quarrying, exploration, stonemasonry, construction, ceramics, foundries, brick manufacture and heavy clay, industrial mineral and the production and the use of silica sand and flour (Cherrie 2009, p. 98). A considerable number of Australian workers are employed in these sectors (NOHSC 1993). Workers exposed to RSC have the potential to develop silicosis, a disease characterized by fibrosis of the lungs (AIOH 2009, p.9).
RSC has also been linked to the development of lung cancer, autoimmune disorders and chronic renal disease (Key-Schwartz 2003, p. 260).
Cyclone with a cassette has been used to sample airborne silica (Key-Schwartz 2003, p. 260). A sampling of dust containing silica is a highly complex process that requires strict adherence to standardized methodologies. In Australia, AS 2985(2004) based on ISO 7708: 1995 is used (AIOH 2009, p.8). Further analysis for the major forms of RSC can be carried out by X-ray diffraction spectrophotometry (XRD), infrared absorption spectrometry (IR) and colourimetric spectrophotometry (Key-Schwartz et al. 2003, p.268). The colourimetric method is the least precise of the three (Key-Schwartz 2003, p.272).
Only XRD and IR are common in Australia (AIOH 2009, p.8). The AIOH recommends long-hour sampling for RSC typically 8-hour or 12-hour time-weighted average exposure and analysis by an accredited laboratory. RSC concentration of 0.05 mg/m3 for an 8-hour sampling period is considered to offer an acceptable level of uncertainty while a similar concentration for 4 hours is considered below legal standards (AIOH 2009, p.8). The current exposure standard for RSC in Australia is 0.1 mg/m3 (AIOH 2009, p.14).
The “Guidelines for Health Surveillance” (NOHSC 1995) offers the criterion for carrying out health surveillance for adverse health effects in workers at high risk. It requires surveillance for workers with potential for long-term exposure at concentrations exceeding 50% of the legal exposure limits (AIOH 2009, p. 8).
Control principles advocated by AIOH involve cutting down on a mechanical generation of dust through the use of wet processes and adequate ventilation. Others are dust education, medical monitoring, compliance with regulatory limits and proper PPE (P1 or P2 efficiency half-face respirators).
Mining is a significant economic activity in Australia. This means workers in this industry still face greater exposure to respirable crystalline silica. Studies have linked RSC to adverse health effects such as lung cancer.Though debate still rages on the exact health effects of this mineral, authorities need to strictly enforce compliance with the exposure limits and other control measures so that exposures are reduced to as low as reasonably practical
Conclusion
Exposure to particles and matters of chemicals in the form of gases, vapours, mists and aerosols represents the most serious of hazards at the workplace. These components may enter the body system through inhalation, absorption or ingestion and may result in adverse health effects or even death. Regulations set up by governments are aimed at reducing exposure to dangerous levels of such chemicals. Assessment of exposure is undertaken through various sampling techniques. The sampling results are central to exposure assessment and high standards of quality control should be observed throughout the exercise. The results of sampling will determine the level of exposure and more so the compliance with regulatory laws. Chemical exposure at “safe limits” cannot be relied on. Therefore, other control measures are required at the workplace. These include setting up efficient aeration systems such as ventilation and the proper use of Personal Protective Equipment.
References
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