Back to All Posts

Posted by John Kurowski

How to Properly Select Toxic Gas Monitoring Locations

October 20, 2022

From the Author

 My friend and colleague, Jim Blais, originally wrote the contents of this blog as part of a White Paper. I have decided to break up the original White Paper into four separate blogs that are detailed below. Hallam-ICS will publish one blog per month, for four months. I wanted to get this valuable information back to the forefront of our clients and potential clients.

As Jim Blais originally wrote: Hallam’s controls experience has evolved into a more dedicated passion and focus – for the integrity of safety control system platforms, as they apply to hazardous gas and chemical monitoring; to ensure the safe storage, handling and use of hazardous materials used in research laboratories and semiconductor manufacturing facilities. A small client sample includes Applied Materials, Akoustis, ASML, Boston University, Cornell University, Global Foundries, IBM, MIT, MIT -Lincoln Laboratories, and many others.


Jim Blais originally developed this section of his white paper to provide best practices through years of studying codes, attending (SESHA) seminars and meeting with other safety professionals. All of us at Hallam-ICS hope that this will serve to answer questions you may have or stir thought that will lead to creative approaches to TGMS’ in the future. This blog will cover only the third topic below. (The first two topics were published in previous months) Hallam-ICS will publish the remaining topic in next month.

  1. Harmonizing Code and Best Practices for a TGMS
  2. Key Elements of a Safety Requirement Specification (SRS)
  3. Selection of Toxic Gas Monitoring Detection Technologies
  4. How to Properly Select Toxic Gas Monitoring Locations 


How to Properly Select Toxic Gas Monitoring Locations

In addition to assessing required gas detection technologies, their capabilities (and limitations) related to safe detection of the specified hazardous gas target, proper placement and mounting location of each hazardous gas monitoring sensor is equally critical to protecting people, equipment and the facility.

Equipment Exhaust Monitoring 

The ideal equipment exhaust duct penetration and mounting locations for gas detection (where mechanical exhaust features permit) should include the installation of an exhaust duct sensor or extractive tubing adapters as near as possible to the following mounting locations. These locations are found to achieve optimum gas sensor detection with respect to gas leak mixing and local flow behavior within the duct:

Duct Features

 These ideal exhaust duct locations address the potential for “pocketing” of a gas leak. Pocketing is when an accumulation of a gas collects in a section of a ventilated enclosure and may occur if the linear velocity and mixing of airflow within the enclosure is not uniform and the gas detector is located down inside the equipment enclosure (gas cabinet, VMB, GIB).

While these monitoring locations do not eliminate potential gas pocketing from occurring, they greatly increase the likelihood of detection within the primary exhaust (i.e., the cabinet exhaust duct work). In normal operation, the primary exhaust provides the safe pathway of the “materials of concern” to carry the gas leak away from the equipment cabinet and occupied work area.

Ambient Breathing Zone Monitoring

The need for continuous ambient breathing zone gas monitoring seems subject to debate. Many facilities provide ambient monitoring and many do not. We recommend that a risk assessment be applied for each application using hazardous production materials (HPMs) addressing how each hazardous gas is stored and delivered from “source” to “use” point?

Are all of the unwelded hazardous gas piping connection points and gas process components (transducers, regulators, valves, etc.) contained within a properly ventilated and exhausted housing? These include gas cabinets, valve manifold boxes, gas isolation boxes, tool cabinets and vacuum pump housings. For any unwelded HPM gas piping connection point and component not enclosed and properly ventilated, exhausted and monitored, the risk of a potential gas leak exists.

An Example Illustrating the Challenges of Ambient Monitoring

The following outlines an actual hazardous gas incident that promotes the need for applying ambient gas monitoring, while also illustrating the challenges associated with placement of effective ambient gas monitoring.

The following occurred within a semiconductor fabrication facility, configured with a cleanroom bay and adjoining process equipment service chase bay arrangements.

As is typical, the service chase included diverse tool support equipment, including vacuum pumps. In this instance; the vacuum pump did not have a pump containment housing and cabinet exhaust. In the event of a vacuum pump “seal failure,” hazardous gas escapes into the service chase ambient breathing zone.

The hazardous gas in this vacuum pump seal incident was arsine (AsH3), with an ACGIH TWA-TLV of only 5 ppb. The relative vapor density of AsH3 is 2.7 (air = 1), with poor odor warning properties (garlic like), it immediately begins to collect near the floor (due to the service chase ventilation flow and equipment location arrangements).

The service chase was equipped with ambient hydride gas monitoring, but with extractive sample tubing heights located at 7 to 8 feet above finished floor (so as not to interfere with periodic equipment maintenance activities).

Upon an actual leak, no detection or alarms were activated until a maintenance tool technician entered the service chase and walked through the vacuum pump leak area. This movement caused the “pooled” arsine gas near the floor to rise up around his pathway and up to an extractive gas sample tube – enabling detection, interlocks and alarming to finally occur.

Key points contributing to a lack of gas detection include:

  1. No containment of the tool vacuum pump via an exhausted enclosure and cabinet exhaust gas monitoring.
  2. Service chase ventilation did not permit the capture of the heavier than air leaking arsine gas into a chase exhaust ventilation airflow stream for detection and removal.
  3. Gas monitoring sample locations focused only on “lighter than air” hydride gas targets.

Ambient gas sensor mounting locations are therefore selected with consideration given to:

  1. Target gas properties (i.e., relative vapor density)
  2. Optimum locations related to the potential / possible leak points. For example, near tool gas cabinet enclosure openings (possible escaping pocketed gas), non-enclosed vacuum pumps or other exposed piping connections in the service chase or other assessed locations for maintenance activities.
  3. Area ventilation and ambient airflow streams and the potential changes to ventilation flows created by equipment location changes.

Case Study: Major Semiconductor ManufacturerEffective ambient gas monitoring in selected tool service chase areas can also provide supplemental protection via notification alerts to employees involved in periodic maintenance activities (e.g., chamber cleaning, implant source housing cleaning, etc.), where potentially hazardous exposures, which cannot reasonably be controlled by other means may occur.

OSHA “Breathing Zone”

Considering the OSHA definition of a “breathing zone” as being an area within 10 inches of a workers nose or mouth; this suggests the height of a typical working employee either standing or sitting would be most representative of a “typical breathing zone”.

Using the SEMI S8 “Safety Guideline for Ergonomics Engineering of Semiconductor Manufacturing Equipment” as a reference for determining the typical working “breathing zone heights”:

  1. For standing individuals (eye height) placed between 53” and 69”
  2. For seated individuals (eye height) placed between 26” and 33”

Oxygen Deficiency Hazards

Similar continuous ambient breathing zone monitoring for safe levels of oxygen should also always be considered, examined and applied if a gas storage area, laboratory or room lacks adequate ventilation rates that cannot protect against a worst case release of large quantities of an inert gas.

  • Inert gases such as nitrogen, helium, argon, carbon dioxide, krypton, neon, and xenon provide no warning properties and have different expansion ratios; they may initially be heavier than air or immediately lighter than air (Helium).

A risk assessment considering the potential worst case leak scenario, the room area and available room ventilation calculations should be performed to ensure that there is no potential leak scenario that would create an atmosphere of less than 19.5% oxygen (OSHA/ACGIH definition of O2 deficiency).

If there is any question continuous ambient breathing zone oxygen monitoring should be applied with local and remote alarm notification, activating room/area evacuation and or no entry at <19.5% O2.


We have tried to provide you in a few pages with a sound foundation based upon some basic, fundamental elements when designing a Toxic Gas Monitoring system; with this blog focusing on properly selecting gas detection locations for your application. We are very passionate about this topic and take a great deal of satisfaction in the work that we do, to help others keep workplaces safe for their employees!

What’s Next?

I hope this white paper has helped you to take your first steps towards designing or improving the hazardous gas monitoring system at your facility.

Please contact us if you have any questions or are looking for help on how to design, install, program, commission and maintain a Toxic Gas Monitoring System.


About the Author

John has a B.S.  in Chemical Engineering from Penn State and a M.S. in Manufacturing Engineering from RPI. Prior to coming to Hallam-ICS, John had 30 years of experience working in Manufacturing/Process/Facilities Engineering for printed circuit board and semiconductor manufacturing.  As a Senior Engineer at Hallam-ICS, John is responsible for the design and specification of Toxic Gas Monitoring Systems. He participates in all project design phases from concept through construction documents.

Read   My Hallam Story  

About Hallam-ICS

Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina and our projects take us world-wide.

Contact Us

Topics: Toxic Gas Monitoring Systems

John Kurowski

By John Kurowski October 20, 2022

Back to All Posts

Subscribe to Email Updates


Management of Change Protocol for a Chemical or High Tech Facility

Toxic Gas Monitoring: Cross-Sensitivity or Cross-Interference

Do OSHA and NFPA Care About Toxic Gas Monitoring?

Reduce your Infrared Thermography costs by up to 25% Get a Quote
7 Reasons why TGMS and FAS should communicate Webinar Recording Access