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The development of a portable diode laser based gas sensor and its application to sensitive, selective, on-line monitoring of formaldehyde concentrations present in a catalytic Trace Contaminant Control System (TCCS) in a 5-day period in August 1999 is reported. The TCCS was originally developed for the Lunar-Mars Life Support Test program in 1996-1997 at NASA-JSC. The motivation for monitoring H2CO levels in a sealed human rated environment is that its presence can cause headaches, throat and ear irritation at low concentrations (>100 ppb), and more serious adverse effects at higher concentration levels. Consequently, NASA has established a spacecraft maximum allowable H2CO concentration of 40 ppb for crew exposure for a 7 to 180 days period [1].