In the United States, entry into permit-required confined spaces must comply with regulations promulgated by the Occupational Safety and Health Administration (OSHA). These regulations include developing a written program, issuing entry permits, assigning attendant(s), designating entrants, and ensuring a means of rescue. According to the OSHA, a permit-required confined space (permit space) has the three characteristics listed above (which define a confined space) and one or more of the following: Contains or has the potential to contain a hazardous atmosphere Contains a material that has the potential for engulfing the entrant Has an internal configuration that might cause an entrant to be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross section Contains any other recognized serious safety or health hazards. In addition to the hazards posed by the design of the space, work activities can also pose serious safety hazards (heat, noise, vapors, etc.) that must be taken into account when identifying safety measures that must be taken. Atmospheric hazards The most common hazard seen in confined spaces is that of atmospheric hazards. These affect air quality and present immediate hazards to health or life. Acceptable atmospheric conditions must be verified before entry, and must be monitored continuously while the space is occupied. The oxygen concentration, the presence of toxic gases, and flammable material are the three conditions that must be monitored. According to OSHA, oxygen concentration is considered safe if it is between 19.5% and 23.5% of the total atmosphere. To protect against toxic gases, contaminants have permissible exposure limits (PELs), which are set by OSHA within the United States. Work also cannot continue if the concentration of a material reaches or exceeds 10% of its lower explosive limit. Even if a tank or similar vessel initially is tested and found to contain breathable air, a hazard can develop during operations inside the tank if residues inside the tank can release toxic gas or vapor when disturbed or if accidentally ignited. Steel water tanks may have dangerously low oxygen concentration when the interior rusts. Entry certification In many situations, certification of non-hazardous atmosphere by a trained or competent person is required before personnel may enter a confined space without the use of a respirator. In the United States Navy, that person is the designated shipboard gas-free engineer. In the maritime sector an NFPA Certified Marine Chemist is