Control of Nitrous Oxide During Cryosurgery

Compressed gases such as nitrous oxide (N2O) are often use d to obtain the cold temperatures

needed for cryosurgery. Cryosurgical instruments which use compressed gas are designed to

allow the gas to expand through a valve inside the metal tip of the cryosurgical probe, causing the

tip to reach extremely low temperatures. If the exhaust gas from the probe is improperly vented,

N2O concentrations in air can reach several thousand parts per million during a cryosurgical

procedure; and depending on the room ventilation rate, levels may remain elevated for long

periods of time following the procedure. Exposures should be minimized to prevent short-term

behavioral and long-term reproductive health effects that can be caused by N2O.

NIOSH stud ies have shown that air borne con cen tra tions of N2O from cryosurgical units can be

ef fec tively con trolled by ven ti la tion and equip ment main te nance, and if this is not pos si ble,

through sub sti tu tion with a less haz ard ous cryo genic gas.

O VENTILATION

Most mod ern N2O cryosurgical units are equipped with a scav eng ing port where the gas is ex -

hausted (see Fig ure 1). Some older units not equipped with scav eng ing can be retro fit ted by the

man u fac turer to in clude a scav eng ing port. A length of plas tic tub ing avail able from the man u facturer can be con nected di rectly to this port, with the other end con nected to a dis charge lo ca tion.

There are var i ous op tions for how to dis charge the N2O. You should con sult with the man u facturer to de ter mine which scav eng ing meth ods they rec om mend for their equip ment. Some of the

more com mon dis charge meth ods used in clude the fol low ing:

CONTROLS

HC 29
Ex haust Di rectly to Out doors: Since the gas is un der con sid er able pres sure, the dis charge end 

of the tub ing can be ex hausted di rectly out doors. This can be ac com plished by plac ing the end of

the hose out of a win dow or at tached to a pipe placed through a wall or ceil ing to the out doors.

Care should be taken to en sure that the dis charge lo ca tion is not an area where peo ple are normally pres ent or in close prox im ity to an out side air in take or win dow where N2O could re-enter

the build ing.

Ex haust into a Nonrecirculating Gen eral Ven ti la tion Sys tem: If a nonrecirculating gen eral

ven ti la tion sys tem is pres ent, the hose can be placed up in an ex haust air vent and the N2O will

be dis charged out of the build ing. Care should be taken to en sure that the hose is placed far

enough into the duct sys tem to pre vent N2O from re-entering the room. To make sure that it is

not, air mon i tor ing should be con ducted. Dis charge hoses should not be placed in re cir cu lat ing

gen eral ven ti la tion duct work since this could dis trib ute the N2O to other ar eas of the build ing.

Ex haust to Vac uum Scav enging Sys tem: In some in stances, the dis charge hose can be

hooked to a vac uum sys tem for re moval of the N2O. A qual i fied in di vid ual should de ter mine if

an ex ist ing waste an es thetic gas dis posal sys tem or a med i cal-surgical vac uum source can be

used for this pur pose. The high ex haust flowrates from some cyrosurgical units (up to 90 li ters

per min ute) may be greater than the ca pac ity of the vac uum sys tem or leave min i mal vac uum

ca pac ity for other parts of the sys tem. Due to the high gas pres sure, a spe cially de signed in terface may be needed be tween the vac uum sys tem and the cryo genic scav eng ing tube to pre -

vent back pres sur iza tion. Since N2O is an ox i dizer, its cor ro sive ef fect on vac uum sys tem

com po nents and com pat i bil ity with ma te ri als and lu bri cants used in the vac uum sys tem should

also be con sid ered.

Cryo probe

Cryotips

Pressure

gauge block

Scavenging

port

Hose to

exhaust

N O

cylinder

2

Figure 1. Cryosurgical system.
O EQUIPMENT MAINTENANCE

Even with a proper scav eng ing unit, leaks can oc cur that can cause high lev els of N2O in the work 

area. The equip ment man u fac turer should be con sulted to de ter mine what rou tine main te nance

is nec es sary for the par tic u lar cryo genic unit. Pe ri odic leak test ing of all fit tings and con nec tions

should be con ducted to en sure this source of ex po sure is min i mized. A di rect read ing N2O mon itor or a soap so lu tion can be used for this pur pose.

O SUBSTITUTION

If scav eng ing is not pos si ble, the user should con sider sub sti tu tion of car bon di ox ide (CO2),

which can also be used as a cryo genic gas on many of the cryosurgical units. CO2 is con sid ered

less haz ard ous and has a much higher air borne ex po sure limit than N2O. How ever, since it does

not cool as low as N2O (− 79 °C for CO2 vs. − 89 °C for N2O), it will likely re quire lon ger freeze

times. The cryosurgical unit man u fac turer should be con sulted for guid ance re gard ing the use of

CO2 with their equip ment. Ad di tionally, ex perts in the med i cal field should be con sulted con cern -

ing the im pli ca tions of chang ing to CO2 as the freeze agent as this may not be a med i cally vi a ble

op tion in all cases.

 For More Information

To ob tain more free in for ma tion about con trol ling this haz ard or for in for ma tion about other oc cupa tional safety and health is sues:

*NIOSH is the Fed eral agency re spon si ble for con duct ing re search and mak ing rec om men da tions for pre vent ing work-related

ill nesses and in ju ries. All HAZARD CONTROLS are based on re search stud ies that show how worker ex po sures to

haz ard ous agents or ac tiv i ties can be sig nif i cantly re duced.

 Acknowledgments

The prin ci pal con tri bu tors to this pub li ca tion are Wil liam J. Daniels, Charles S. McCammon, and

Jerome P. Flesch of NIOSH and Lee Shands, In dian Health Ser vice. The draw ing is by Dick

Carlson. Desk top pub lish ing was pro vided by Kristina Dragon and Su san Kaelin.

January 1999 DHHS (NIOSH) Publication No. 99–105

— call NIOSH* at 1–800–35–NIOSH (1–800–356–4674) , or

— visit the NIOSH Homepage on the World Wide Web at

http://www.cdc.gov/niosh

This document is in the public domain and may be freely copied or reprinted. NIOSH encourages

all readers of this HAZARD CONTROLS to make it available to all interested employers and workers.