under normal conditions. fire and explosion hazards exist when concentrated oxidants and fuels are brought together in close proximity, although an ignition event (e.g., heat or spark) is needed to trigger combustion. concentrated oxygen will allow combustion to proceed rapidly and energetically. steel pipes and storage vessels used to store and transmit both gaseous and liquid oxygen will act as a fuel; and therefore the design and manufacture of oxygen systems requires special training to ensure that ignition sources are minimized. highly concentrated oxygen in a high-pressure environment can spontaneously ignite hydrocarbons such as oil and grease, resulting in a fire or explosion. the heat caused by rapid pressurization serves as the ignition source. for this reason, storage vessels, regulators, piping and any other equipment used with highly concentrated oxygen must be "oxygen-clean" prior to use to ensure the absence of potential fuels. this does not only apply to pure oxygen; any concentration significantly higher than atmospheric (approximately 21%) carries a potential ignition risk. some hospitals have instituted "no-smoking" policies which can help keep ignition sources away from medically piped oxygen. these policies do not eliminate the risk of injury among patients with portable oxygen systems, especially among smokers. other potential sources of ignition include candles, aromatherapy, medical equipment, cooking, and deliberate vandalism. delivery various devices are used for oxygen administration. in most cases, the oxygen will first pass through a pressure regulator, used to control the high pressure of oxygen delivered from a cylinder (or other source) to a lower pressure. this lower pressure is then controlled by a flowmeter (which may be preset or selectable) which controls the flow at a measured rate (e.g., litres per minute ). the typical flowmeter range for medical oxygen is between 0 and 15 lpm with some units capable of obtaining up to 25 lpm. many wall flowmeters using a thorpe tube design are able to be dialed to "flush" oxygen which is beneficial in emergency situations. low-dose oxygen many people only require slight increases in inhaled oxygen, rather than pure or near-pure oxygen. these requirements can be met through a number of devices dependent on situation, flow requirements, and personal preference. a nasal cannula (nc) is a thin tube with two small nozzles inserted into a person's nostrils. it can provide oxygen at low flow rates, 1–6 litres per minute (lpm), delivering an oxygen concentration of 24–40%. there are also a number of face mask options, such as the simple face mask, often used at between 5 and 10 lpm, capable of delivering oxygen concentrations between 35% and 55%. this is closely related to the more controlled air-entrainment masks, also known as venturi masks, which can accurately deliver a predetermined oxygen concentration from 24 to 50%. in some instances, a partial rebreathing mask can be used, which is based on a simple mask, but features a reservoir bag, which can provide oxygen concentrations of 40–70% at 5–15 lpm. demand oxygen delivery systems (dods) or oxygen resuscitators deliver oxygen only when the person inhales or the caregiver presses a button on the mask (e.g., nonbreathing patient). these systems greatly conserve oxygen compared to steady-flow masks, and are useful in emergency situations when a limited supply of oxygen is available and there is a delay in transporting the person to higher care. due to utilization of a variety of methods for oxygenation requirements performance differences arise. they are very useful in cpr, as the caregiver can deliver rescue breaths composed of 100% oxygen with the press of a button. care must be taken not to over-inflate the person's lungs, for which some systems employ safety valves. these systems may not be appropriate for people who are unconscious or in respiratory distress because of the required respiratory effort. high flow oxygen delivery for patients requiring high concentrations of oxygen, a number of devices are available. the most commonly utilized device is the non-rebreather mask (or reservoir mask). non-rebreather masks draw oxygen from attached reservoir bags with one-way valves that direct exhaled air out of the mask. if flow rate is not sufficient (~10l/min), the bag may collapse on inspiration. this type of mask is indicated for acute medical emergencies. the delivered fio2 (inhalation volumetric fraction of molecular oxygen) of this system is 60–80%, depending on oxygen flow and breathing pattern. another type of device is a humidified high flow nasal cannula which enables flows exceeding a person's peak inspiratory flow demand to be delivered via nasal cannula, thus providing fio2 of up to 100% because there is no entrainment of room air. this also allows the person to continue to talk, eat, and drink while still receiving therapy. this type of delivery method is associated with greater overall comfort, improved oxygenation, respiratory rates and reduced sputumstatis compared with face mask oxygen. in specialist applications such as aviation, tight-fitting masks can be used. these masks also have applications in anaesthesia, carbon monoxide poisoning treatment and in hyperbaric oxygen therapy. positive pressure delivery patients who are unable to breathe on their own will require positive pressure to move oxygen into their lungs for gaseous exchange to take place. systems for delivery vary in complexity and cost, starting with a basic pocket mask adjunct which can be used to manually deliver artificial respiration with supplemental oxygen delivered through a mask port. many emergency medical service members, first aid personnel, and hospital staff may use a bag-valve-mask (bvm), which is a malleable bag attached to a face mask (or invasive airway such as an endotracheal tube or laryngeal mask airway), usually with a reservoir bag attached, which is manually manipulated by the healthcare professional to push oxygen (or air) into the lungs. this is the only procedure allowed for initial treatment of cyanide poisoning in the uk workplace.pin-indexed oxygen regulator for portable d-cylinder, usually carried in an ambulance's resuscitation kitautomated versions of the bvm system, known as a resuscitator or pneupac can also deliver measured and timed doses of oxygen directly to people through a facemask or airway. these systems are related to the anaesthetic machines used in operations under general anaesthesia that allow a variable amount of oxygen to be delivered, along with other gases including air, nitrous oxide and inhalational anaesthetics. drug delivery oxygen and other compressed gases are used in conjunction with a nebulizer to allow delivery of medications to the upper and/or lower airways. nebulizers use compressed gas to propel liquid medication into therapeutically sized aerosol droplets for deposition to the appropriate portion of the airway. a typical compressed gas flow rate of 8–10 l/min is used to nebulize medications, saline, sterile water, or a combination these treatments into a therapeutic aerosol for inhalation. in the clinical setting, room air (ambient mix of several gasses), molecular oxygen, and heliox are the most common gases used to nebulize a bolus treatment or a continuous volume of therapeutic aerosols. exhalation filters for oxygen masks filtered oxygen masks have the ability to prevent exhaled particles from being released into the surrounding environment. these masks are normally of a closed design such that leaks are minimized and breathing of room air is controlled through a series of one-way valves. filtration of exhaled breaths is accomplished either by placing a filter on the exhalation port or through an integral filter that is part of the mask itself. these masks first became popular in the toronto (canada) healthcare community during the 2003 sars crisis. sars was identified as being respiratory based, and it was determined that conventional oxygen therapy devices were not designed for the containment of exhaled particles. in 2003, the hiox80 oxygen mask was released for sale. the hiox80 mask is a closed design mask that allows a filter to be placed on the exhalation port. several new designs have emerged in the global healthcare community for the containment and filtration of potentially infectious particles. other designs include the iso-o2 oxygen mask, the flo2max oxygen mask, and the o-mask. typical oxygen masks allow a person to breathe in a mixture of room air and therapeutic oxygen. however, as filtered oxygen masks use a closed design that minimizes or eliminates the person's contact with and ability to inhale room air, delivered oxygen concentrations in such devices have been found to be elevated, approaching 99% using adequate oxygen flows. because all exhaled particles are contained within the mask, nebulized medications are also prevented from releasing into the surrounding atmosphere, decreasing the occupational exposure to healthcare staff and other people. aircraft in the united states, most airlines restrict the devices allowed on board an aircraft. as a result, passengers are restricted in what devices they can use. some airlines will provide cylinders for passengers with an associated fee. other airlines allow passengers to carry on approved portable concentrators. however, the lists of approved devices varies by airline so passengers may need to check with any airline they are planning to fly on. passengers are generally not allowed to carry on personal cylinders. in all cases, passengers need to notify the airline in advance of their equipment. effective may 13, 2009, the department of transportation and faa ruled that a select number of portable oxygen concentrators are approved for use on all commercial flights. faa regulations require larger airplanes to carry d-cylinders of oxygen for use in case of an emergency. oxygen conserving devices since the 1980s, devices have been available which conserve stored oxygen by delivering it during the portion of the breathing cycle when it is more effectively used. this has the effect of stored oxygen lasting longer, or a smaller, and therefore lighter, portable oxygen delivery system being practicable. this class of device can also be used with portable oxygen concentrators, making them more efficient. the delivery of supplemental oxygen is most effective if it is made at a point in the breathing cycle when it will be inhaled to the alveoli, where gas transfer occurs. oxygen delivered later in the cycle will be inhaled into physiological dead space, wher it serves no useful purpose as it cannot diffuse into the blood. oxygen delivered during stages of the breathing cycle in which it is not inhaled is also wasted. a continuous constant flow rate uses a simple regulator, but is inefficient as a high percentage of the delivered gas does not reach the alveoli, and over half is not inhaled at all. a system which accumulates free-flow oxygen during resting and exhalation stages, (reservoir cannulas) makes a larger part of the oxygen available for inhalation, and it will be selectively inhaled during the initial part of inhalation, which reaches furthest into the lungs. a similar function is provided by a mechanical demand regulator which provides gas only during inhalation, but requires some physical effort by the user, and also ventilates dead space with oxygen. a third class of system (pulse dose oxygen conserving device, or demand pulse devices) senses the start of inhalation and provides a metered bolus, which if correctly matched to requirements, will be sufficient and effectively inhaled into the alveoli.such systems can be pneumatically or electrically controlled. adaptive demand systems a development in pulse demand delivery are devices that automatically adjust the volume of the pulsed bolus to suit the activity level of the user. this adaptive response in intended to reduce desaturation responses caused by exercise rate variation. pulsed delivery devices are available as stand alone modules or integrated into a system specifically designed to use compressed gas, liquid oxygen or oxygen concentrator sources. integrated design usually allows optimisation of the system for the source type at the cost of versatility. transtracheal oxygen catheters are inserted directly into the trachea through a small opening in the front of the neck for that purpose. the opening is directed downward, towards the bifurcation of the bronchi. oxygen introduced through the catheter bypasses the dead spaces of the nose, pharynx and upper trachea during inhalation, and during continuous flow, will accumulate in the anatomic dead space at the end of exhalation and be available for immediate inhalation to the alveoli on the following inhalation. this reduces wastage and provides efficiency roughly three times greater than with external continuous flow. this is roughly equivalent to a reservoir cannula. transtracheal catheters have been found to be effective during rest, exercise and sleep. long term oxygen therapy