enters (inwardly converging walls, or a floor

that slopes downward to a smaller section);

or it has any other serious safety or health

hazards.

Verification Testing

Before a permit space that may have a hazardous atmosphere can be entered, the

atmosphere must be tested using the steps

identified on the permit (developed during

evaluation testing). Verification testing is

done to make sure that the chemical hazards

that may be present are below the levels

necessary for safe entry, and that they meet

the conditions identified on the permit. Test

the atmosphere in the following order: (1) for

oxygen, (2) for combustible gases, and then

(3) for toxic gases and vapors.2 The testing

results -- the actual test concentrations --

must be recorded on the permit near the levels identified for safe entry.

Duration of Testing

For each test required on the permit, you

must allow enough time for the air from the

space to be drawn into the equipment and

for the sensor (or other detection device) to

react to the chemical if it is present. This is

considered the “minimum response time”

and it will be noted by the manufacturer in

the operator’s manual. Be aware that you

will need to add time to this "minimum

response time" if you have attached hosing

or a probe extension to the inlet. The additional time is needed to allow the air from

the different depths of the space to be pulled

into the equipment inlet.

A confined space is one that is large

enough to enter and perform assigned work

in; it has limited or restricted ways to enter

or exit the space; and it was not designed to

be occupied continuously by a worker.

Evaluation testing

The atmosphere within a confined space

must be tested using equipment that is

designed to detect the chemicals that may

be present at levels that are well below the

defined exposure limits. Evaluation testing

is done to:

• determine what chemical hazards are or

may become present in the space’s atmosphere, and

• identify what steps must be followed and

what conditions must be met to ensure

that atmospheric conditions are safe for a

worker to enter the space.

The testing results and the decisions about

what steps must be followed before entry

must be evaluated by, or reviewed by, a

technically qualified professional like an

OSHA consultation service, a certified

industrial hygienist, a registered safety

engineer, or a certified safety professional.

The technically qualified professional must

consider all of the serious hazards in his/her

evaluation or review.

A permit space is a confined space that has

one or more of the following features: it has

or may contain a hazardous atmosphere; it

contains a material that can engulf a person

who enters; it has an inside design that

could trap or asphyxiate a person who

FactSheet

Procedures for Atmospheric Testing 

in Confined Spaces1

Atmospheric testing is required for two distinct purposes: evaluation of the 

hazards of the permit space and verification that acceptable conditions exist for

entry into that space.
Testing Conditions in Spaces that May

Have Layered Atmospheres

For permit spaces that are deep or have

areas leading away from the entry point, the

atmosphere may be layered or may be different in remote areas. For these spaces, testing must be done in the area surrounding

the worker, which is considered four (4) feet

in the direction of travel and to each side. If

a sample probe is used to do the testing,

This is one in a series of informational fact sheets highlighting OSHA programs, policies or 

standards. It does not impose any new compliance requirements. For a comprehensive list of 

compliance requirements of OSHA standards or regulations, refer to Title 29 of the Code of Federal

Regulations. This information will be made available to sensory impaired individuals upon request.

The voice phone is (202) 693-1999; teletypewriter (TTY) number: (877) 889-5627.

then the worker must move slowly enough

so that testing is completed, keeping the

equipment “response time” in mind, before

he/she moves into the new area.

Retesting the Space During Entry or

Before Re-Entry

Test the permit space routinely to make sure

that the atmospheric conditions continue to

be safe for entry.3

U.S. Department of Labor

www.osha.gov

(800) 321-OSHA

For more complete information:

DSTM 9/2005

1Title 29 Code of Federal Regulations 1910.146, Appendix B.

2 29 CFR 1910.146(c)(5)(ii)(C) and (d)(5)(iii).

3 29 CFR 1910.146(c)(5)(ii)(F) and (d)(5)(ii).