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Atmospheric Testing in Confined Spaces

You cannot see, smell or feel whether air is safe to breathe. This guide explains why testing exists and why it belongs to trained people, with pointers to official guidance.

Why does the air in a confined space need testing?

The air in a bin, silo, pit or tank can be unsafe even when it looks normal. It may be short of oxygen, or it may contain gases that harm or kill. Conditions can also change as work goes on.

Testing is the way to find out. It is a skill and a responsibility, not a casual check.

A casual check by a coworker is not a substitute for proper testing.

• Air may lack oxygen
• Gas can build up quietly
• Conditions can change mid-job

Can you tell by smell or by how you feel?

No. Some harmful gases have no smell. Others have a smell that fades because the nose gets used to it. Feeling fine at the start does not mean the air will stay safe.

A person can become confused or collapse with very little warning. This is why you should never rely on your senses.

Some gases are heavier than air and sink, while others rise. Where you stand when you test matters, which is why training is needed. Even people with years of experience can be overcome, because the body gives little warning that the air has changed.

• No smell does not mean safe
• A smell can disappear quickly
• Feeling fine is not a test

Who should do the testing?

Testing should be done by people who are trained to use the equipment and to understand the results. Instruments need to be suitable for the gases of concern, kept in working order and used in the right way.

This site does not give limits, readings or settings. Official sources and the instrument manufacturer provide them. Never use a number from memory.

Instruments need regular care and checks that the manufacturer describes. An instrument in poor condition can give a false sense of safety.

• Never copy numbers from memory
• Use the instrument maker's instructions

When and where is testing done?

Official guidance describes testing before entry and, where needed, during the work. Different parts of a space can have different conditions, so testing in one spot may not be enough.

Training will cover how to test safely from outside, and what to do with the results. Ask OSHA, NIOSH or a recognized training provider.

Stop work and leave if anything changes, such as new odors, dizziness or an alarm, then call for help from a safe place.

• Test from outside first where training covers it
• Do not rely on one reading in one spot

What if the air is not safe?

If a test shows a problem, entry should not go ahead until the cause is handled in line with official guidance, and the air is tested again. Sometimes the safest decision is to not enter at all.

Never decide to go in anyway because the job is urgent. Get expert help.

Ventilation can help, but the manufacturer and official guidance describe when and how it is used. Do not assume it works without testing. Write down what you found, who tested and what was decided. Records help show that the decision was based on information rather than guesswork.

• Stop entry
• Handle the cause per official guidance
• Test again
• Get expert help if unsure

What about farm spaces such as manure pits and silos?

These spaces can contain gases from decomposing material, and the hazards may be greater at certain times. See the related guides on manure pit entry and silo gas.

Extension services, including Penn State Extension, publish farm-specific information on dangerous gases.

Lagoons, pits and silos each have different gas patterns, and extension programs can describe them.

• Decomposing manure and silage give off gases
• Hazards change with time and activity
• Use farm-specific extension guidance

Where can you learn more?

OSHA and NIOSH publish confined space information, and extension programs and safety organizations offer hands-on training with real instruments.

If someone collapses in a confined space, call 911. Do not go in. Rescuers without protection often become victims too.

Ask whether training near you includes a chance to use instruments in a practice setting. If a farm uses a hired confined space team, ask how they test and what training they hold. Training providers can also show how to keep records of each test.

Last reviewed: November 12, 2026. Written by the Farm Safety Editorial Team. See our editorial policy.