Considerations for Hazardous Locations in General Industry
Navigating Electrical Hazards
Industrial hygienists often need to work in or around unfamiliar locations that may present electrical hazards. These locations fall under OSHA regulation 29 CFR 1910.307, Hazardous (Classified) Locations, of Subpart S, Electrical. This article provides basic information about the classification of these hazardous spaces, commonly referenced standards, and other sources of information. CLASSIFICATION OF HAZARDOUS LOCATIONS Hazard classification by 29 CFR 1910.307 is intended for managing the electrical equipment used in locations that contain, or may contain, material that is flammable or combustible in the air. The classification of the area involves a separate process from the classification of the flammable or combustible material itself. Per 29 CFR 1910.307, OSHA mandates that classification and selection of equipment be performed “under the supervision of a qualified registered professional engineer.” The regulation explains that classification of the space “depend[s] on the properties of the flammable vapors, liquids or gases, or combustible dusts or fibers that may be present therein and the likelihood that a flammable or combustible concentration or quantity is present.” It is required that “each room, section, or area” be considered individually.
Classification is based on the type of hazard and its severity. The terms associated with classification are defined in Subpart S, 29 CFR 1910.399. OSHA allows two classification systems: the class/division system and the class/zone system. There are three classes based on the general type of hazard. Flammable gases and vapors are designated Class I, combustible dusts are Class II, and ignitable fibers and “flyings” are Class III. No definition of flyings appears in the OSHA regulation, but the 2019 version of NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, describes flyings as solids, such as fibers, that can suspend in the air and settle under their own weight, and are larger than 500 µm nominal. In the class/division system, the three classes are further separated into Division 1 and Division 2 to indicate modes for hazardous conditions. Division 1 designates areas where the hazard is present normally. Division 2 areas are those where the hazard is present only under abnormal conditions.  The class/zone system is an alternative to the class/division system. While OSHA currently recognizes only three zones in 29 CFR 1910.307 (Zone 0, 1, and 2), an additional three zones (Zone 20, 21, and 22) are recognized by many organizations IHs tend to be familiar with, including Underwriters Laboratories, ANSI, the International Society of Automation, and NFPA.  PERTINENT NFPA STANDARDS OSHA defers to NFPA 70, National Electrical Code (NEC), for guidelines regarding the type and design of electrical equipment and installations. OSHA does not reference a particular revision of NFPA 70 in 29 CFR 1910.307. The current edition is 2020.  Article 500 of NFPA 70 mirrors OSHA in requiring classification based on properties of materials that could be present in the area and “the likelihood that a flammable or combustible concentration or quantity is present.” Independent classification of each room, section, or area is also required. Sections of the standard specific to situations or industries will provide details about which electrical area classifications can abut. Information pertaining to dual classification—the use of both the class/zone system and  he class/division system in a single facility—is included. Overlapping of classification is generally not allowed. NFPA 70 makes use of the “qualified person” concept. For example, when a zone classification system is implemented, according to NFPA 70, “classification of areas, engineering and design, selection of equipment and wiring methods, installation, and inspection” must be conducted by qualified persons. Thus, if the organization is following this standard requirement, even inspection elements must be performed by a person recognized as qualified.  The 2020 edition of NFPA 70 incorporates several new references. In particular, IHs may be interested in reviewing the “Combustible Gas Detection System” section. The 2020 NFPA 70 references the 2018 revision of the American Petroleum Institute’s Recommended Practice for Classification of Locations for Electrical Installations of Petroleum Facilities Classified as Class I, Zone 0, Zone 1 or Zone 2 (API RP 505). NFPA 70 is therefore more up to date than the OSHA regulation, which still refers to the 1997 version of the API standard. NFPA 70 references two important documents IHs should be familiar with if involved with locations classified as hazardous. These documents are NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, and NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas. The current revision of NFPA 497 and NFPA 499 (2017) both recommend implementing the guidance “with sound engineering judgment.” NFPA 497 and NFPA 499 have sections dedicated to procedures for classifying that make use of engineering judgment.  While NFPA provides guidelines for classification, a process is desired in order to meet the challenge of classification. The Process Industry Practices organization provides a process to approach classification in the 2016 revision of PIP ELEHA01, Engineering Guide for Determining Electrical Area Classification, which “describes an engineering work process for gathering the necessary information and applying the applicable codes and standards to develop a well-documented area classification” for “areas containing flammable liquids, gases, or vapors and combustible dust.” This guide is useful because it suggests which industry standard should be used for assessing area classification—for example, it explains when to apply NFPA 497, NFPA 499, or API RP 505.  OSHA RECOMMENDED STANDARDS OSHA recommends several references within 29 CFR 1910.399 as guides for determining when flammable gases or vapors are present continuously or for long periods of time. Unfortunately, the references are woefully out of date. Table 1 can help readers find the modern equivalents. The table is especially useful for IHs who need expertise to work closely with engineers. The recommended references are for flammable gases and vapors; additional standards exist for dusts and fibers—see ISA12.10-1988, Area Classification in Hazardous (Classified) Dust Locations, and ANSI/ISA-12.00.02-2009 (R2014), Certificate Standard for AEx Equipment for Hazardous (Classified) Locations
Table 1. OSHA-Recommended References and Their Modern Counterparts
Tap on the table to open a larger version in your browser.
The ISA 60079 document has subsections, such as ISA 60079-0, Explosive Atmospheres - Part 0: Equipment - General Requirements. OSHA lists ISA 60079-related subsections which are recognized as part of “Appropriate Test Standards” listed by OSHA in the webpage for the Nationally Recognized Testing Laboratory Program (NRTL). For example, a recognized standard listed is ISA 60079-0, Explosive Atmospheres - Part 0: Equipment - General Requirements. In addition to ISA standards, at the NRTL webpage, there are UL standards related to “Hazardous (Classified) Locations.” Process safety engineers also use the references in Table 1. IHs are encouraged to consult with these professionals for their expertise.  DOCUMENTATION An IH faced with a location classified as hazardous should take advantage of the fact that these locations require documentation. The documentation should include information about the hazard and the hazard state, and which standards were followed when the area was classified. It might also include maps and schematics, controls (for example, the use of intrinsically safe electrical power, ventilation diagrams, and fire suppression systems), and company policies. The OSHA regulation requires the involvement of registered professional engineers, so contacting the engineering staff at the facility is the ideal way to access updated documentation. Facility and safety personnel could also be good starting points for inquiries about documentation. In 29 CFR 1910.307(b), OSHA mandated that designations after August 2007 for locations classified as hazardous under the class/zone system and the class/division system need documentation that “shall be available to those authorized to design, install, inspect, maintain, or operate electric equipment at the location.” This general requirement is echoed by NFPA 70. NFPA 497 requires specific documentation and that it be kept current. Prudent practices for any IH should include verifying the accuracy of critical information when safety is a concern. IHs working in and around locations classified as hazardous can point to an OSHA interpretation letter pertaining to the need for accurate classification drawings. In a December 12, 1997, Standard Interpretation related to 29 CFR 1910.307, Electrical Hazardous Area Classification Drawings in the Workplace, OSHA indicated that such drawings distributed to employees must be current to provide for their safety.  OSHA REFERENCE TO GROUPS IHs would likely seek hazard-specific information pertaining to groups of gases, vapors, and dusts, as defined in NFPA 70. Information on groups can be found within NFPA 70, NFPA 497, and NFPA 499. The class/division system includes groups A through G. The class/zone system includes designations IIA, IIB, IIC, IIIA, IIIB, and IIIC. For more details, see chapter 5 of NFPA 70. (An explanation of the various definitions used in the classification is beyond the scope of this article.)  IHs are encouraged to review chapter 4 of NFPA 497, which provides useful information for select chemicals such as CAS number, classification, and data useful to common IH calculations. Chapter 5 of NFPA 499 provides information about groups of select combustible dusts. NEW REALMS 29 CFR 1910.307 regulates electrical devices, electrical installation components, and electrical machinery that is at risk of interacting with hazardous material. Implementation of the regulation to manage the electrical hazard is generally under the supervision of a qualified registered professional engineer.  IHs may find themselves involved with these locations due to their expertise with hazardous material. To successfully navigate in these realms, which are primarily under the control of engineers, and to make full use of documentation, IHs should familiarize themselves with the standards discussed in this article.    VERONICA STANLEY, MSPH, CIH, CSP, CESCP, works as adjunct faculty in Workforce Development at Harrisburg Area Community College and is a PhD student at Indiana University of Pennsylvania. She can be reached via LinkedIn. Acknowledgment: The Synergist thanks Christopher A. Janicak, PhD, CSP, CEA, ARM, a professor in the Department of Safety Sciences at Indiana University of Pennsylvania, for reviewing a draft of this article. Send feedback to The Synergist.
Free Online Access to NFPA Standards
NFPA’s codes and standards are accessible for free online to individuals who create an account on For the free online version of NFPA 70, users can navigate directly to relevant chapters and view the document one page at a time. Relevant definitions can be found in NFPA’s Glossary of Terms.
Which Code Does Your Facility Use?
Different localities have adopted different versions of NFPA 70, the National Electrical Code. To determine which version is in effect in your area, view this page on NFPA's website.
American Petroleum Institute: API RP 505, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2, 2nd edition (August 2018). ANSI: ANSI/ISA 61241-0 (12.10.02)-2006 (R2015), Electrical Apparatus for Use in Zone 20, Zone 21, and Zone 22 Hazardous (Classified) Locations - General Requirements (2015). Energy Institute: Model Code of Safe Practice, Part 15: Area Classification for Installations Handling Flammable Fluids, 4th edition, (PDF, June 2015). IAEI Magazine: “Understanding the Zone Area Classification Method in the NEC” (November 2002). International Society of Automation: ISA12.10-1988, Area Classification in Hazardous (Classified) Dust Locations (1988). National Fire Protection Association: NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (2017). National Fire Protection Association: NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas (2017). OSHA: Standard Interpretations: Electrical Hazardous Area Classification Drawings in the Workplace (December 1997). OSHA: Underwriters Laboratories, Inc.: Grant of Expansion of Recognition, (December 2016). Process Industry Practices: PIP ELEHA01, Engineering Guide for Determining Electrical Area Classification (PDF, September 2016). UL: UL 60079-25, Explosive Atmospheres - Part 25: Intrinsically Safe Electrical Systems (2012).
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Disadvantages of being unacclimatized:
  • Readily show signs of heat stress when exposed to hot environments.
  • Difficulty replacing all of the water lost in sweat.
  • Failure to replace the water lost will slow or prevent acclimatization.
Benefits of acclimatization:
  • Increased sweating efficiency (earlier onset of sweating, greater sweat production, and reduced electrolyte loss in sweat).
  • Stabilization of the circulation.
  • Work is performed with lower core temperature and heart rate.
  • Increased skin blood flow at a given core temperature.
Acclimatization plan:
  • Gradually increase exposure time in hot environmental conditions over a period of 7 to 14 days.
  • For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment on day 1 and a no more than 20% increase on each additional day.
  • For workers who have had previous experience with the job, the acclimatization regimen should be no more than 50% of the usual duration of work in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4.
  • The time required for non–physically fit individuals to develop acclimatization is about 50% greater than for the physically fit.
Level of acclimatization:
  • Relative to the initial level of physical fitness and the total heat stress experienced by the individual.
Maintaining acclimatization:
  • Can be maintained for a few days of non-heat exposure.
  • Absence from work in the heat for a week or more results in a significant loss in the beneficial adaptations leading to an increase likelihood of acute dehydration, illness, or fatigue.
  • Can be regained in 2 to 3 days upon return to a hot job.
  • Appears to be better maintained by those who are physically fit.
  • Seasonal shifts in temperatures may result in difficulties.
  • Working in hot, humid environments provides adaptive benefits that also apply in hot, desert environments, and vice versa.
  • Air conditioning will not affect acclimatization.
Acclimatization in Workers