Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Weld Leak Check and He
Name: Shadi B.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A

I am conducting a weld leak check using Helium gas. However, I don't know what the harmful effects of Helium on the operators on the work floor, I know it can cause Asphyxia if inhalled right out.

I am thinking of changing from Helium to regular atmosphere, but I know there are some concerns between the "He" molecule size and the Atmosphere (different gases) size. can you tell me what the biggest molecule size in our atmosphere is ("H","O","N", "He"??) and based on that I will be able to decide weather to use Atmosphere insted of Helium to conduct my leak check. note that the leak check is being conducted at a well ventilated area. (shop floor environment)

Helium gas is essentially non-toxic except, as you point out, if it excludes oxygen. It would be difficult to imagine any realistic conditions that this would occur in a lab or work floor environment.

You would not be able to substitute another gas for helium in a helium leak detector because the operating principles of the instrument presumes that helium is the gas probe. No gas that would be presently "naturally" could or would work because the atmospheric gasses are always de-gassing from any vacuum system.

Your concern, as a matter of principle, is to be commended. But given the environment you describe, any hazard is absolutely negligible.

Vince Calder

As long as the leak check is conducted in a well-ventilated environment, helium is an excellent gas to use. Helium is certainly the smallest in size of all the gases. It is certainly much smaller than any of the components of air (N2, O2, CO2, Ar). So Helium will leak the fastest, making it the best operational choice for a leak check. Helium is also not poisonous.

Its only health risk is that it can displace air, lowering the oxygen content of what your workers might breathe. Other gases, such as argon and nitrogen, do this too. In addition, helium rises rather than sinks, so it will not acccumulate in most confined spaces. Tha main advantage I could see for using air instead of helium in your situation is that helium is fairly expensive, while air is essentially free. But if the performance improvement from helium outweighs its cost, any safety concerns should not be significant.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

I would not worry about it. Helium is inert. Divers breathe the stuff (mixed with oxygen, of course) on purpose to avoid the bends.

Anyway, the bigger of the two most abundant molecules in air is N2 (0.31 nanometers). Next is O2 (0.29 nm).

Tim Mooney

Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory