Tuesday, July 2, 2013

The Problems of Radiation

Selection of a Reference Level

The ICRP recommends an annual reference level of 10 mSv effective dose from radon, above which doses should be considered "occupational" and require radiological control intervention [ICRP 2007]. Assuming continuous occupancy in a work year (2,000 hours), the reference level is equivalent to the NIOSH REL of 1 WLM∙y–1

. Additionally, the reference level can be equated to
a continuous workshift-averaged PAEC value of 1.7 × 10–3 mJ∙m–3 (0.083 WL) and a radon concentration of about 800 Bq∙m–3 (22 pCi∙L–1) assuming 40% equilibrium (Feq=0.4). This appears to be in reasonable agreement with thresholds for posting and controls specified by the major U.S. regulatory agencies [10 CFR 835; 10 CFR 20; 29 CFR 1926.53] as discussed in the Appendix. Note that the recommended reference level is based on 2,000-hour occupancy, which is not feasible for mine closure activities because of inclement weather conditions during the winter. Assuming that work is limited to a 9-month period (1,500 hours) the reference PAEC is about 2 × 10–3 mJ∙m–3 (0.1 WL), and the radon concentration is about 1 kBq∙m–3 (27 pCi∙L–1 as shown in Table 2.

The reference level establishes a threshold for intervention under conservative assumptions. In this case, workplace controls are considered if a worker’s annual effective dose from radon exposure is likely to exceed 10 mSv if left unmitigated. Compliance with the reference level is assured by average PAEC concentrations less than 0.1 WL and yearly occupancy no more than 1,500 hours. Current estimates from occupancy records suggest that individuals involved in mine closure activities spend far less than 1,500 hours per year at the work site because of funding constraints, procurement regulations, and other work preparation activities. Reference concentration values may be adjusted upward on the basis of improved occupancy estimates and assurances that the annual effective dose goal of 10 mSv (1 WLM) is maintained.

State inactive mine reclamation program staff had established ALARA goals for mine reclamation work and have incorporated these goals into its most recent revision to mine safety procedures included in contract specifications. The state inactive mine reclamation program goals are similar to, but slightly less conservative than, the proposed reference level. The state inactive mine reclamation program goals are not to exceed 15 mSv effective dose from radon and 20 mSv dose-equivalent from external irradiation. On the basis of an assumed annual occupancy of 800 hours, a DCF of 8.6 mSv∙WLM–1, and allowing for about 10% error, state inactive mine reclamation program staff equate the radon ALARA goal to a PAEC concentration of 0.33 WL (1.55 WLM∙y–1), which is used to trigger respiratory protection. If one assumes the NIOSH-preferred DCF (excluding error), then the state inactive mine reclamation program radon goal reduces to 1.5 WLM∙y–1. Conversely, applying the 800 hour occupancy to the NIOSH-recommended reference level of 1 WLM∙y–1 results in a PAEC concentration of 0.2 WL.

Monitoring results and onsite observations during this HHE suggest that ionizing radiation hazards during mine closure activities are relatively low overall; however, radon exposures necessitating intervention can occur at some work locations. Limiting occupancy, simple engineering controls (i.e., barriers, ventilation), and the use of respiratory protection in some/ certain situations are the preferred control measures for keeping radon exposures ALARA. Extreme differences in observed radon concentrations between and within adits suggests that knowledge of environmental factors (moisture, ventilation patterns, weather conditions) and a rigorous monitoring plan are necessary for appropriate hazard characterization. Employers and employees together must remain vigilant in identification, characterization, and mitigation of radon hazards in the workplace through training, monitoring practices, and application of control measures. Low-level gamma radiation fields at the surface of waste rock piles and near mine adits also may contribute to the ionizing radiation hazard; however, on the basis of our observations, intervention is rarely needed and may be limited to the case of reducing exposures to the declared pregnant worker.

On the basis of our findings, we recommend the actions listed below to create a more healthful workplace. Our recommendations are based on the hierarchy of controls approach (Appendix). This approach groups actions by their likely effectiveness in reducing or removing hazards. In most cases, the preferred approach is to eliminate hazardous materials or processes and install engineering controls to reduce exposure or shield employees. Until such controls are in place, or if they are not effective or feasible, administrative measures and/or personal protective equipment may be needed.

This hierarchy can be summarized as follows:

  1. Elimination
  2. Substitution
  3. Engineering controls
  4. Administrative controls
  5. Personal protective equipment

Control methods at the top of the list are potentially more effective and protective than those at the bottom. Following the hierarchy normally leads to the implementation of inherently safer systems, ones where the risk of illness or injury has been substantially reduced.

Adopt a reference level or ALARA goal for requiring intervention (i.e., engineering, administrative, or PPE controls). For example, a reference level of 10 mSv effective dose from radon progeny in a working year is consistent with the recommendations of the ICRP and existing NIOSH recommendations for radon exposures in uranium miners. This reference level is well below regulatory limits but is readily achievable in occupational settings associated with mine closure activities.

2. Increase sample frequency and/or duration to improve the precision of exposure estimates. If CWLMs are used, then, in addition to lengthening sampling periods, the instruments should be tested and/or calibrated in a radon chamber
Occupational Exposure Limits and HealthEffects

In evaluating the hazards posed by workplace exposures, NIOSH investigators use both mandatory (legally enforceable) and recommended OELs for chemical, physical, and biological agents as a guide for making recommendations. OELs have been developed by federal agencies and safety and health organizations to prevent the occurrence of adverse health effects from workplace exposures. Generally, OELs suggest levels of exposure that most employees may be exposed to for up to 10 hours per day, 40 hours per week, for a working lifetime, without experiencing adverse health effects. However, not all employees will be protected from adverse health effects even if their exposures are maintained below these levels.

In the United States, OELs have been established by federal agencies, professional organizations, state and local governments, and other entities. Some OELs are legally enforceable limits, while others are recommendations. The U.S. Department of Labor OSHA PELs (29 CFR 1910 [general industry]; 29 CFR 1926 [construction industry]; and 29 CFR 1917 [maritime industry]) are legal limits enforceable in workplaces covered under the Occupational Safety and Health Act of 1970. NIOSH RELs are recommendations based on a critical review of the scientific and technical information available on a given hazard and the adequacy of methods to identify and control the hazard. NIOSH also recommends different types of risk management practices (e.g., engineering controls, safe work practices, employee education/training, personal protective equipment, and exposure and medical monitoring) to minimize the risk of exposure and adverse health effects from these hazards.

 Other OELs that are commonly used and cited in the United States include the TLVs recommended by ACGIH, a professional organization. The TLVs are developed by ACGIH committee members from a review of the published, peer-reviewed literature. ACGIH TLVs are not consensus standards. TLVs are considered voluntary exposure guidelines for use by industrial hygienists and others trained in this discipline "to assist in the control of health hazards" [ACGIH 2011].

Employers should understand that not all hazardous chemicals and physical agents have specific OSHA PELs, and for some agents the legally enforceable and recommended limits may not reflect current health-based information. However, an employer is still required by OSHA to protect its employees from hazards even in the absence of a specific OSHA PEL.

OSHA requires an employer to furnish employees a place of employment free from recognized hazards that cause or are likely to cause death or serious physical harm [Occupational Safety and Health Act of 1970 (Public Law 91–596, sec. 5(a)(1))]. Thus, NIOSH investigators encourage employers to make use of other OELs and exposure guidelines when making risk assessments and risk management decisions to best protect the health of their employees. NIOSH investigators also encourage the use of the traditional hierarchy of controls approach to eliminate or minimize identified workplace hazards. This includes, in order of preference, the use of (1) substitution or elimination of the hazardous agent, (2) engineering controls (e.g., local exhaust ventilation, process enclosure, dilution ventilation), (3) administrative controls (e.g., limiting time of exposure, employee training, work practice changes, medical surveillance), and (4) personal protective equipment (e.g., respiratory protection, gloves, eye protection, hearing protection). Control banding, a qualitative risk assessment and risk management tool, is a complementary approach to protecting employee health that focuses resources on exposure controls by describing how a risk needs to be managed. Information on control banding is available at http://www.cdc.gov/niosh/topics/ctrlbanding/. This approach can be applied in situations where OELs have not been established or can be used to supplement the OELs, when available.Page 20

Below we provide OELs and exposure guidelines for alpha and gamma radiation, as well as a discussion of the potential health effects from exposure to ionizing radiation.

 Radon is a colorless, odorless, inert, radioactive noble gas that has three isotopic forms found ubiquitously in nature: 222Rn, which is a member of the 238U decay chain; 220Rn (commonly known as "thoron"), which is in the decay chain of 232Th; and 219Rn (known as "actinon"), which results from the decay of 235U. Of the three forms, 222Rn and its subsequent radioactive decay products present the greatest risk in most environmental and occupational settings because of its natural abundance. 222Rn undergoes radioactive decays via a series of solid short-lived radionuclides (i.e., polonium-218, lead-214, bismuth-214, and polonium-214), commonly referred to as "radon progeny" or "radon daughters." These decay products appear either as unattached ions or are attached to condensation nuclei or dust particles, forming a respirable radioactive aerosol.

Environmental levels of radon in the United States vary widely, with average indoor concentrations in U.S. homes of about 46 Bq∙m–3 and in Colorado homes of 96 Bq∙m–3 [Marcinowski et al. 1994]. Outdoor radon concentrations tend to be much lower with national and regional (Nevada and Colorado) averages of about 15 Bq∙m–3 [Price et al. 1994; Borak and Baynes 1999], but progeny equilibrium is typically greater outdoors [NCRP 2009]. NCRP estimates that radon progeny exposure accounts for about 36% of the total dose received by the U.S. population annually [NCRP 2009]. The main contributor to tissue absorbed dose is densely ionizing radiation in the form of alpha particles from the decay of respired short-lived radon progeny; therefore, the organ most at risk from exposure is the lung, primarily from deposition of radon progeny in the bronchial epithelium. Dose to other organs and the fetus from inhaled radon progeny are at least an order of magnitude less than that of the lung [Kendall and Smith 2002]. Numerous studies of underground uranium miners who were exposed to relatively high levels of radon have unequivocally established radon as a human lung carcinogen [IARC 1988]. EPA claims that radon is the second leading cause of lung cancer in the United States and is the leading cause among persons who never smoked. The estimated risk from lifetime exposure at the EPA action level of 150 Bq∙m–3 (4 pCi∙L–1) is 2.3% [EPA 2003].

Much less information is available on other health outcomes associated with radon exposure. There is sparse evidence suggesting increased leukemia in uranium miners exposed to radon [Darby et al. 1995; Rericha et al. 2006] although most miner studies have not shown similar results [Tomasek et al. 1993; Laurier et al. 2004; Mohner et al. 2006; Schubauer-Berigan et al. 2009; Lane et al. 2010]. Some researchers have postulated that radon progeny that is deposited on skin surfaces can result in non-negligible dose to sensitive basal cells, which may result in increased incidence of non-melanoma skin cancer [Sevcova et al. 1978; Eatough and Henshaw 1991; Denman et al. 2003]. The current weight of evidence is insufficient to establish a causal link between radon and skin cancer in humans [Charles 2007a; Charles 2007b].