Editing Environmental control system (section)

== Pressurization ==
{{main|Cabin pressurization}}
[[File:Outflow Valve and Pressure Relief Valve on a Boeing 737-800.jpg|thumb|right|Outflow and pressure relief valve on a [[Boeing 737 Next Generation|Boeing 737-800]]]]
Airflow into the fuselage is approximately constant, and pressure is maintained by varying the opening of the out-flow valve (OFV). Most modern jetliners have a single OFV located near the bottom aft end of the fuselage, although some larger aircraft like the Boeing&nbsp;747 and 777 have two.

In the event the OFV should fail closed, at least two positive pressure relief valves (PPRV) and at least one negative pressure relief valve (NPRV) are provided to protect the fuselage from over- and under- pressurization.

Aircraft cabin pressure is commonly pressurized to a cabin altitude of 8000&nbsp;feet or less. That means that the pressure is {{convert|10.9|psi}}, which is the ambient pressure at {{convert|8000|ft}}. Note that a lower cabin altitude is a higher pressure. The cabin pressure is controlled by a cabin pressure schedule, which associates each aircraft altitude with a cabin altitude. The new airliners such as the [[Airbus A350]] and [[Boeing 787]] will have lower maximum cabin altitudes which help in passenger fatigue reduction during flights.

The atmosphere at typical jetliner cruising altitudes is generally very dry and cold; the outside air pumped into the cabin on a long flight has the potential to cause [[condensation]] which might in turn cause corrosion or electrical faults, and is thus eliminated. Consequently, when humid air at lower altitudes is encountered and drawn in, the ECS dries it through the warming and cooling cycle and the water separator mentioned above, so that even with high external relative humidity, inside the cabin it will usually be not much higher than 10% relative humidity.

Although low cabin humidity has health benefits of preventing the growth of [[fungus]] and [[bacteria]], the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to [[dehydration]], leading to fatigue, discomfort and health issues. In one study the majority of flight attendants reported discomfort and health issues from low humidity.<ref>{{cite book |url=https://books.google.com/books?id=pFds11oY8QEC |editor-first=Niren Laxmichand |editor-last=Nagda |title=Air Quality and Comfort in Airliner Cabins |publisher=ASTM International |year=2000 |isbn=978-0-8031-2866-8}}</ref> In a statement to [[US Congress]] in 2003 a member of the Committee on Air Quality in Passenger Cabins of Commercial Aircraft said "low relative humidity might cause some temporary discomfort (e.g., drying eyes, nasal passages, and skin), but other possible short- or long-term effects have not been established".<ref>{{cite report |chapter-url=http://www7.nationalacademies.org/ocga/testimony/Aircraft_Cabin_Environment.asp |title=Cabin Air Quality |archive-url=https://web.archive.org/web/20080621121806/http://www7.nationalacademies.org/ocga/testimony/Aircraft_Cabin_Environment.asp |archive-date=2008-06-21 |df=dmy-all |first=William W. |last=Nazaroff |chapter=Statement of William W. Nazaroff, Ph.D. Professor of Environmental Engineering, University of California, Berkeley and Member, Committee on Air Quality in Passenger Cabins of Commercial Aircraft |date=5 June 2003 |website=nationalacademies.org |publisher=National Academy of Sciences |place=Washington, DC}}</ref>

A cabin humidity control system may be added to the ECS of some aircraft to keep relative humidity from extremely low levels, consistent with the need to prevent condensation.<ref>{{cite web |url=http://findarticles.com/p/articles/mi_m0CWU/is_2007_March_5/ai_n27164656 |title=CTT Systems AB receives cabin humidity control system order from Jet Aviation AG |publisher=Airline Industry Information |date=5 March 2007}}</ref> Furthermore, the Boeing&nbsp;787 and Airbus&nbsp;A350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.