gas, vapours, mists and dusts). Entire Vapour space of storage tank. In the US, the independent National Fire Protection Association (NFPA) publishes several relevant standards, and they are often adopted by government agencies. May 2019 Electric equipment depending on the protection techniques described by paragraph (g)(3)(i) of this section may not be suitable for use at temperatures lower than -20 C (-4 F) unless they are approved for use at lower temperatures. area in which an explosive gas atmosphere is likely to occur occasionally in normal operation. Pit will be considered as Zone 1. Category 2 equipment may be used in zone 1 or zone 2 areas. It uses the familiar Class/Division system and adapts the IEC/CENELEC Zones into it while maintaining NEC wiring methods and . Class II combustible dusts are grouped into one of three (3) groups: E combustible metal dusts, F combustible carbonaceous dusts or G other combustible dusts. A Zone defines the general nature - if it is a gas or dust - and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. Then, the level of risk of an explosion is based on the frequency and duration of the occurrence of an explosive atmosphere. This equipment is intended for use in Zone 1 (gases) or Zone 21 (dusts). August 2018 Beside the Lower Explosion Limit, each substance also has an Upper Explosion Limit (UEL). The zone classification for gases is divided into three zones, namely Zone 0, Zone 1 and Zone 2 and for dusts Zone 20, Zone 21 and Zone 22. Zone 1 - Explosive atmospheres are likely to occur. March 2021 May 2022 Zone 2 Explosive Mixtures not likely to occur /occur short duration in normal operation (lt10 Hrs/ Yr). When working in or specifying equipment for use in hazardous environments, we see a lot of confusion in the industry regarding the different certifications and requirements. For equipment provided with threaded entries for NPT threaded conduit or fittings, listed conduit, conduit fittings, or cable fittings shall be used. Zone defines the general nature - if it is a gas or dust - and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. This is because Zone 0 usually classifies areas for gas storage or containment, and any lighting required would be used on a small and portable basis for visual inspection, e.g. The design of the equipment must ensure protection, even in the event of rare incidents relating to the equipment. A special standard for instrumentation is IEC/EN 60079-27, describing requirements for Fieldbus Intrinsically Safe Concept (FISCO) (zone 0, 1 or 2) (This special standard has been withdrawn, and has been partially replaced by: IEC/EN60079-11:2011 and IEC/EN60079-25:2010)[1], A special standard for instrumentation is IEC/EN 60079-27, describing requirements for Fieldbus Non-Incendive Concept (FNICO) (zone 2) (This special standard has been withdrawn, and has been partially replaced by: IEC/EN60079-11:2011 and IEC/EN60079-25:2010)[9]. Group is divided in three groups where Group I is reserved for mining locations. In other cases, the atmosphere is normally non-hazardous, but a dangerous concentration can be reasonably foreseensuch as operator error or equipment failure. I Flammable gases or vapors may be present, 1 Ignitable concentrations of hazards exist under normal operation conditions and/or where the hazard is caused by frequent maintenance or repair work or frequent equipment failure, E Combustible metal dusts: aluminum, commercial alloys and magnesium, III Easily ignitable fibers or flyings may be present, 0 Ignitable concentrations of flammable gases or vapors which are present continuously or for long periods of time, 1 Ignitable concentrations of flammable gases or vapors which are likely to occur under normal operating conditions; may exist frequently because of repair/maintenance operations or leakage; or equipment is operated in a manner that equipment breakdown/faulty operations could result in the release of ignitable concentrations of flammable gases or vapors and failure of the equipment, 2 Ignitable concentrations of flammable gases or vapors which are not likely to occur under normal operating conditions and if they do will only persist for a short period of time; or volatile flammable liquids, gases, or vapors are confined within closed containers/systems and can escape only as a result of an accidental rupture; or volatile flammable liquids, gases, or vapors are normally prevented by positive mechanical ventilation, but may become hazardous due to system failure, 20 Combustible dusts or ignitable fibers/flyings are present continuously or for long periods of time, 21 Combustible dusts or ignitable fibers/flyings are likely to occur under normal operating conditions; or may exist frequently because of repair/maintenance operations or leakage; or equipment is operated in a manner that breakdown/faulty operations could result in the release of ignitable concentrations of combustible dust or fibers/flyings and failure of the equipment, 22 Combustible dusts or ignitable fibers/flyings are likely to occur under normal operating conditions; or may exist frequently because of repair/maintenance operations or leakage; or equipment is operated in a manner that breakdown/faulty operations could result in the release of ignitable concentrations of combustible dust or fibers/flyings and failure of the equipment, Mines susceptible to firedamp (flammable mixture of gases naturally occurring in a mine, Explosive dusts Group III equipment is subdivided into three subgroups, A Atmospheres containing acetone, ammonia, ethyl alcohol, gasoline, methane, propane, or gases/vapors of equivalent hazard, A Atmospheres containing solid particles and fibers greater than 500 microns that could be suspended in air and settle out, B Atmospheres containing acetaldehyde, ethylene, or gases / vapors of equivalent hazard, B Atmospheres containing combustible dusts other than combustible metal dusts, C Atmospheres containing acetylene, hydrogen, or gases/vapors of equivalent hazard, C Atmospheres containing combustible metal dusts. a headlamp, rather than a dedicated fixed luminaire. National Electric Code 2020 edition National Fire Protection Association. Temperature Control ), Group F Carbon-based (eg. The Zone system has wider use in the chemical and petrochemical industries. NEC 2 Divisions (Div 1 2) Division I Comprising of Area Same as Zone 0 1. A: When comparing Divisions and Zones, Division 2 is equivalent to Zone 2, but Division 1 is equivalent to either Zone 0 or 1. You will find these zones at the manifold of a tank truck or the interior of a silo. Beside sufficient fuel, also sufficient oxygen is needed for combustion of the mixture. In electrical and safety engineering hazardous locations HazLoc pronounced hazlk are places where fire or explosion hazards may exist. The key difference between Divisions and Zones relate to terminations. These would be then labeled with each of the individual methods. In the IEC standards hazardous areas are classified into zones based upon the frequency and duration of an explosive atmosphere. 1) MESG (Maximum Experimental Safe Gap) - The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration. Groups E, F, and G are for dusts and flyings (Class II or III). 2: For the requirements for electrical and electronic. In the United States, the most commonly followed system is the NEC (National Electric Code), while the rest of the world generally follows the IEC (International Electrotechnical Commission). October 2020 The Class/Division/Group system is based on Article 500 of the National Electrical Code (NEC) where. As a result, all hazardous area equipment must be designed with hazardous area zone classifications in mind, as the zone governs the level of protection and precaution required. NEGATIVE PRESSURE In Europe the ATEX 114 Directive (2014/34/EU) is applicable to equipment that is used in hazardous area's. That is why the use of mobile phones on petrol forecourts are banned. Area Classification. The value is represented is volume percentage of oxygen. | For Gas/Vapor this would be Zone 0. Atmospheres containing propane, acetone, benzene, butane, methane, petrol, hexane, paint solvents or gases and vapors of equivalent hazard. Examples are: production facilities in the food industry, chemical plants, oil & gas, and the pharmaceutical industry etc. For specific industries, the specific codes for that industry are important to use. For example, in the US by the Occupational Safety and Health Administration. We will go deeper into ignition assessments in another blog. Equipment shall be approved not only for the class of location, but also for the ignitable or combustible properties of the specific gas, vapor, dust, or fiber that will be present. Unlike ATEX which uses numbers to define the safety "Category" of equipment, namely (1,2 3), the IEC continued to utilise the method used for defining the safe levels of intrinsic safety namely "a" for zone 0, "b" for zone 1 and "c" for zone 2 and apply this Equipment Level of Protection to all equipment for use in hazardous areas since 2009. Zone type (0/1/2/ 20/21/ 22) Zone extent. Hazardous areas are those defined as areas of potential explosion risk due to the presence of flammable gas or vapor. Zone 1 . We don't save this data. Zone 1: An area in which an explosive atmosphere is likely to occur in normal operation. January 2023 Which system is preferred depends on the users preference, how the areas are classified, and the wiring system used. February 2023 But beside electrical equipment also mechanical equipment must be Ex certified, e.g. Conversely if the same material is heated and handled above its flash-point, the area must be classified for proper electrical system design, as it will then form an ignitable mixture.[5]. For more detailed assessments or for situations where no specific industry standards are available , like the chemical or pharmaceutical sector and also specific installation for oil and gas industries, a more quantitative approach can be used. Classification of areas and selection of equipment and wiring methods shall be under the supervision of a qualified registered professional engineer. Ignitable concentrations of flammable gases or vapors which are likely to occur under normal operating conditions. All equipment in Division 1 areas must have an approval label, but certain materials, such as rigid metallic conduit, does not have a specific label indicating the Cl./Div.1 suitability and their listing as approved method of installation in the NEC serves as the permission. A: Yes. Hazardous area classification is a rigorous method of determining where an explosive environment may be present. AddThis use cookies for handling links to social media. Class I Zone 2 Hazardous Areas. of the autoignition temperature of the hazardous atmosphere. So, it is good to know that when a flammable substance concentration exceeds the LEL, it will not automatically lead to an explosion with the highest Pmax. Both external and internal temperatures are taken into consideration. Flammable liquids with a high flash point are less hazardous than liquids Note to paragraph (g)(5)(ii)(B) of this section: An example of such a required marking is "Class I, Zone 0, AEx ia IIC T6." coal and charcoal), Group G Nonconductive dusts (eg. NEC has created an Americanized version of the IEC/CENELEC Zone system in a stand-alone article (Article 505). The plan may contain the list of chemicals with their group and temperature rating. Marking "EEx d" in Accordance with EN 50 0 14 and 50 018 . Not only are hazardous areas classified by Class (nature of the hazardous material,) and Division . December 2021 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 (as incorporated by reference in 250.198). Gas a. Note to paragraph (g)(4) of this section: Low ambient conditions require special consideration. Groups A, B, C, and D are for gases (Class I only). No. March 2022 The relation between that system and de zone classification system is clarified within the following table (only for gases and liquids are displayed, not for dusts and fibers). 2: Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified), Refrigerant machinery rooms that contain ammonia refrigeration systems and are equipped with adequate mechanical ventilation that operates continuously or is initiated by a detection system at a concentration not exceeding 150 ppm shall be permitted to be classified as "unclassified". IECEx Karandikar Certification Atmosphere containing a flammable gas, a flammable liquid produced vapor or a combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ratio is greater than 0.40 and less than 0.80 - such as carbon monoxide, ether, hydrogen sulfide, morphline, cyclopropane, ethyl, isoprene, acetaldhyde and ethylene or gases of equivalent hazard. >> For Gas Vapor this would be Zone 1. Division II Comprising of Area Same as Zone 2. December 2020 It is good to be aware of the fact that the mixture of flammable gas with oxygen has an optimum point where the combustion is most severe and leads to the highest explosion pressure. And safety must be guaranteed in the case of two faults occurring independently of each other. Gases, Vapors and Mists. Group II is for explosive gases (Zone 0, 1 and 2) and Group III is for explosive dusts (Zone 20, 21 and 22). Equipment is non-incendive or non-sparking. 1.1.2 This recommended practice provides information on specific flammable gases and vapors, flammable liquids . Note to paragraph (g)(5)(ii)(C) of this section: The National Electrical Code, NFPA 70, contains guidelines for determining the type and design of equipment and installations that will meet this provision. To make this even easier to understand, take a look at the diagram of a typical petrol station below where we have identified the areas that are classified as Zone 0, Zone 1 and Zone 2. Electrical equipment installed in such locations can provide an ignition source, due to electrical arcing, or high temperatures. Atmospheres containing combustible flyings. Protecting against fire and explosion is of interest for both personnel safety as well as reliability reasons. An oversimplification of this example is shown in Table 1. November 2022 In Northerm America, instead of the zone classification system, the Class/Division system is used. Other ignition sources can also be present, like static electricity. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Workshop Container, Intelligent pressurised container | MUD logging cabin, Battery energy storage system (BESS) container, Laboratory container | workshop container | Equipment containers, Temporary refuge shelter | Toxic gas refuge | Safe haven, Offshore accommodation cabin | office container, Reefer container | Refrigerated container, Intelligent waste water treatment container, IDENTIFYING ZONE 0, ZONE 1, AND ZONE 2 HAZARDOUS AREAS, Temporary refuge (TR) shelter, toxic gas refuge (TGR), Containerised waste water treatment plant. . Zone 2 - Explosive atmospheres are unlikely to occur or present only infrequently and for a short period only. Division 1 and 2 Zone 0, 1 and 2 A (acetylene) IIC (acetylene & hydrogen) B (hydrogen) C (ethylene) IIB (ethylene) PROCESS OF CONTAINER Please read Google Privacy & Terms for more information about how you can control adserving and the information collected. Where there is potential for an explosive atmosphere, special precautions are needed to prevent fires and explosions. Classification of locations, testing and listing of equipment, and inspection of installation, is typically overseen by governmental bodies. All emergency versions, whether Zone 1 or Zone 2 can also be easily visually identified by their red end cap, encapsulating the emergency battery back-up. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for zone system gas groups. The equipment housing may be sealed to prevent entry of flammable gas or dust into the interior. >> For Gas/Vapor this would be Zone 0. . This means that the electrical equipment is safe for use within these area by applying special protection strategies. >> NEC: Class I, Division 2, Group D. | IEC: Zone 2, Group II. Terminology for both hazards and protective measures can vary. The more hazardous the area to more safety measures have to be taken in order to prevent equipment becoming an effective source of ignition. The temperature marking may not exceed the ignition temperature of the specific gas or vapor to be encountered. This level of risk is represented by classifying the hazardous area as Zone 0, Zone 1 or Zone 2 (for gas, vapour and mist atmospheres) or Zone 21 or Zone 22 for dust atmospheres. ESS Container NFPA 70 NEC uses the familiar Class/Division system and has added an Americanized version of the International Electrochemical Commissions (IEC) IEC Zone system which maintains the NEC wiring methods and protection techniques. * Zone 2 a . But first, we must consider what is likely to cause an explosion in the first place. Accommodation Cabin In North America the suitability of equipment for the specific hazardous area must be tested by a Nationally Recognized Testing Laboratory, such as UL, FM Global, CSA Group, or Intertek (ETL). Zone 1 - A place in which an explosive atmosphere is likely to occur in normal operation occasionally This blog post will focus on the different hazardous location classifications and well follow up with another that discusses how equipment can be constructed and certified for use in these environments. Raytec do not currently offer any lighting solutions for Zone 0. (1) Zone 0. [10][11] Some of these are specifically for hazardous locations: All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied. The following content is only related to flammable substances and not to pyrotechnic substances or other instable substances, among those who can decompose. This page was last edited on 20 January 2023, at 22:32. Flour, wood/sawdust, plastic dust). [1] NEC article 500 describes the NEC Division classification system, while articles 505 and 506 describe the NEC Zone classification system. Requirements for good HAC Drawings and Plans. Identifying Zone 0, Zone 1, and Zone 2 Hazardous Areas. September 2021 Equipment can be designed or modified for safe operation in hazardous locations. Each room, section, or area shall be considered individually in determining its classification. Documentation requirements likewise vary. 1: The term "Class I" was originally included as a prefix to, Informational Note No. The IEC 60079 standard set has been adapted for use in Australia and New Zealand and is published as the AS/NZS 60079 standard set. This is also one of the first steps when composing an Explosion Protection Document. Zones. Class I, Zone 0 or Zone 1 locations may not abut Class I, Division 1 or Division 2 locations. Those guidelines address electric wiring, equipment, and systems installed in hazardous (classified) locations and contain specific provisions for the following: wiring methods, wiring connections; conductor insulation, flexible cords, sealing and drainage, transformers, capacitors, switches, circuit breakers, fuses, motor controllers, receptacles, attachment plugs, meters, relays, instruments, resistors, generators, motors, lighting fixtures, storage battery charging equipment, electric cranes, electric hoists and similar equipment, utilization equipment, signaling systems, alarm systems, remote control systems, local loud speaker and communication systems, ventilation piping, live parts, lightning surge protection, and grounding. If equipment has been approved for use in a Division 1 hazardous location, it can be used in a Division 2 hazardous location, providing it is in the same class and group. The two general approaches are: Several techniques of flame-proofing exist, and they are often used in combination: Equipment has flameproof gaps (max 0.006" (150m) propane/ethylene, 0.004" (100m) acetylene/hydrogen), Equipment can be installed in ANY housing provided to IP54.A 'Zener Barrier', opto-isolator or galvanic unit may be used to assist with certification. Area Classification - Hazardous Locations are classified by the likelihood of an ignitable concentration of combustible material being present. Class I, Zone 0 = Area Classification, AEx = Symbol for equipment built to American specifications, ia = Type of protection designations, IIC = Gas classification group (as required), T6 = Temperature Classification, [46 FR 4056, Jan. 16, 1981; 46 FR 40185, Aug. 7, 1981; 72 FR 7210, Feb. 14, 2007], Occupational Safety & Health Administration. To minimize the risk of these fires or explosions, electrical/electronic equipment must be designed, tested and labeled as being acceptable for use in the areas in which it is installed. The codes and standards used in this process provide guidance for selecting,. Have a think about it next time youre filling up! 10/1/20 2: For Group IIIC combustible dusts or metal combustible fibers/flyings, there shall only be Zone 20 or 21 locations. This is exactly defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. The last condition for an explosion is the presence of an effective ignition source in the direct contact with the fuel/air mixture. 10: For information on electrical resistance trace heating for hazardous (classified), Informational Note No. Equipment used in hazardous area must comply to certain Ex standards. The term equipment is applied both for fixed equipment and mobile equipment that are operated in such area. For example, within the U.K. in the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) and in the U.S. in the Occupational Safety and Health Act of 1970 (OSHA). Such areas include a residence or office where the only risk of a release of explosive or flammable gas would be such things as the propellant in an aerosol spray. February 2019 286 natural gas explosions per year in the U.S. causing substantial damage, severe injury or loss of lives. Area's are divided into zones. Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere. Zone 22 Zone 21 Zone20 A hazardous atmosphere formed by dust cloud in air is not likely to occur in . EN 16985 concerning spray cabins and powder coating cabins, EN 12779 related to extraction systems for hot sawdust and chips, EN 60079-10 series, 2020 version for gas and dust explosion hazard, NPR 7910 series for gas and dust explosion hazard based on EN 60079-10-1, 2015 version, EN 13617 for fuel dispensers at filling stations. Intrinsic safety designs equipment to operate using minimal energy, insufficient to cause ignition. . to EN 1127-1): After the hazard area classification is completed, then the ignition assessment starts. Hazardous Area Classifications D103222X012 Product Bulletin 9.2:001 . This level of risk is represented by classifying the hazardous area as Zone 0, Zone 1 or Zone 2 (for gas, vapour and mist atmospheres) or Zone 21 or Zone 22 for dust atmospheres. The inductance of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks, causing an explosion. If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC. Since the changes to the code, hazardous locations with gases or vapors (Class I) present can be reclassified as Zone 0, 1 or 2. Equipment shall be marked in accordance with paragraph (g)(5)(ii)(A) and (g)(5)(ii)(B) of this section, except as provided in (g)(5)(ii)(C). Ex LIGHTER-THAN-AIR GASES & VAPOURS Vapour density of 0.75 is considered as the boundary between lighter and heavier gases / vapours as a safety measure HA of a leak source located in air Source of hazard 4.5 m. 8.0 m R 4.5 m H In this article we will look at what defines Zone 0, Zone 1 and Zone 2 hazardous area classifications and the considerations for specifying lighting into each area. SIGN UP FOR EMAILGet more great content in your inbox. Adapters shall be used for connection to conduit or NPT-threaded fittings. Zone 0, 1, and 2 locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures. . NEW METHODS FOR HAZARDOUS AREA CLASSIFICATION FOR EXPLOSIVE GAS . 7: For further information on application of electrical, Informational Note No. Zone 0/20: An area in which an explosive atmosphere is present continuously or for long periods or frequently. The a subdivisions have the most stringent safety requirements, taking into account more than one independent component faults simultaneously. So yes, it is important to choose the right model and always wonder what am I assessing?. See Figure S-1 for an explanation of this marking. The classification process requires the participation of operations, maintenance, safety, electrical and instrumentation professionals; and the use of process diagrams, material flows, safety data sheets, and other pertinent documents. The escaping (hot) gases must sufficiently cool down along the escape path that by the time they reach the outside of the enclosure not to be a source of ignition of the outside, potentially ignitable surroundings. A light switch may cause a small, harmless spark when switched on or off. eg Production Area, area surrounding zone 0. Equipment marked as suitable for IIB is also suitable for IIA but NOT for IIC. SEWAGE TREATMENT These areas are classified solely for the purpose of ensuring the safe and proper specification and installation of electrical/electronic equipment. Which Standards to use for Hazardous Area Classification. Area which are not devided into Atex zones, are called non-hazardous area's. Class / Division System In Northerm America, instead of the zone classification system, the Class/Division system is used. Intrinsic safety -- This protection technique is permitted for equipment in the Class I, Zone 0 or Zone 1 locations for which it is approved. In some cases, it must be certified as meeting that standard. THE CURRENT STANDARD . Home About Us News Hazardous Environment Classifications: NEC vs IEC, Posted March 11, 2019 by springercontrols. Industrial electrical equipment for hazardous area has to conform to appropriate parts of standard: IEC-60079 for gas hazards, and IEC-61241 for dust hazards. . September 2019 April 2021 The International Electrotechnical Commission publishes the 60079 series of standards[2] which defines a system for classification of locations, as well as categorizing and testing of equipment designed for use in hazardous locations, known as "Ex equipment". 11: For information on electric skin effect trace heating for hazardous (classified), Informational Note No. Class 1/Div 2 ATEX Zone 2 Computers. October 2019 Magnesium, Titanium), IIB Nonconductive dusts (eg. . area in which an explosive gas atmosphere is present continuously, or for long periods, or frequently (more than 10% of the time). Dn 2 - a Ce d e r . Q: Can I use a piece of equipment approved for use in a Division 1 hazardous location in a Division 2 hazardous location? Equipment in this category is intended for use in Zone 0 areas (gases) or Zone 20 (dusts), and must ensure a very high level of protection (i.e. lk) are places where fire or explosion hazards may exist. Flameproof "d" - This protection technique is permitted for equipment in the Class I, Zone 1 locations for which it is approved. Offshore Accommodation Module Methane has a UEL of 17 vol%, which means when the concentration of methane in air is above the 17 vol% an explosion cannot occur even when an ignition source is present. The Hazardous Area Classification presents in this guide is based on the following items as reference: API: API RP 505 Recommended Practice for Classification of Locations for Electrical. Explosion protection must be ensured during normal operation. CUTTING SKIPS THE PRODUCTS YOU NEED, April 2022 5: For further information on ventilation, see NFPA 30-2018, Informational Note No. Zone 0, 1 2 . Then, the level of risk of an explosion is based on the frequency and duration of the occurrence of an explosive atmosphere. Zone 1 if gas group & temp. In some cases, the hazardous atmosphere is present all the time, or for long periods. An old British standard used letters to designate zones. Informational Note No. These generally give rise to areas classied as Zones 0, 1, and 2 respectively. 41 gas explosions per year occur within the UK and app. D8.1.3 Hazardous areas are subdivided into Zones 0,1 or 2, the definitions of each cate-gory being as follows: Zone 0 an area in which an explosive gas-air mixture is continuously present or present for long periods. November 2019 This value of this LOC differs per substance and depends also on the inert gas for achieving a low oxygen concentration.