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A short guide to toxic gas detection on board

Avoiding toxic gases on board is crucial for crew safety. Thankfully, it’s less complicated than you might think.


There can be hundreds of toxic gases on board a ship. Stemming from either your cargo or the vessel’s internal chemical compounds, you might encounter them in a wide range of scenarios. They can’t be eliminated from the ship altogether, but you can put in place simple measures to neutralise the threat. 


What Makes a Gas Toxic? 

When we deal with toxic gases, we’re concerned with three factors: 

  1. A compound’s specific toxicity
  2. Concentration level 
  3. Duration of exposure


Some gases are considered toxic when the TWA level (Time-Weighted Average) exceeds a certain limit. The TWA applies to an eight hour period. Other gases are considered toxic for shorter exposure times. 

Short-Term Exposure Limit (STEL) is used for 15-minute exposures. TWA and STEL limits are unique for each chemical compound and are set by occupational health authorities or safety organizations. Many compounds have both a TWA and a STEL limit. Irrespective of local guidelines, it’s critical that you deal with toxic gases in a safe manner. 


Step by Step: Dealing with Toxic Gases on Board

With good routines, you and your crew can avoid toxic gas exposure. Here are the key steps: 


1. Assess the Cargo and Other Onboard Substances

Consult the Material Safety Data Sheet (MSDS) of the cargo or substance. It will give you an overview of the cargo components and their respective exposure limits. 

It’s also important to map out non-cargo substances on board, such as cleaning agents and lubrication oils. It’s crucial to understand the onboard hazards, for your own and others’ safety.


2. Choose the Appropriate Gas Detection Equipment 

Whenever we engage with a new customer, we start with the same question: What kinds of cargo will your vessel carry? 

As soon as that’s been established, we identify a gas detection setup that will cover all scenarios. If you’re unsure what kinds of cargo you should set up for, look at what your vessel has been carrying the last year. It’s usually a good indicator of what you can expect in the future.


3. Avoid False Positives

Getting reliable readings isn’t just a case of detecting a given gas. Some sensors are designed for instant readings—is it safe to stay here right now?—while others are aimed at long-term, averaged measurements. 

For instance, the concentration of a gas might be well within a given limit—for short time periods. If you use a Gastec tube to evaluate an area, it can indicate that the level of gas is safe at the moment, but it won’t tell you whether you’re safe to occupy that space for hours. 

Repeated sampling with averaging of the values (TWA), would be the right approach. Below is a table that lists the three most common detection methods, and when to use them.  


Indication method

Used for


(what substance)

Quantitative (concentration)

Gastec Tubes


Toxic gases

(single shot)



Electrochemical Sensors (EC)


H2S, CO (and some others) in the PPM range 

STEL or TWA (continuous)



Photoionization Detector (PID)

Photo Ionization

Toxic gases (VOCs) in the PPM range, in air and in inert gases. 

STEL or TWA (continuous)

Multiple, nonspecific



For vessels carrying noxious liquids, your best bet is to use PIDs and Gastec tubes in combination with one another. It gives you the best of two worlds: Identification of the compound (qualitative) and measurement over time (quantitative). 




4. Set Your Alarms Right, and React Accordingly 

Permitted gas concentration levels vary with the territory. Thus, you need to adjust your gas detection alarm settings according to where you operate at any given time. If more than one regulation applies, make sure to comply with the strictest one. 

The most important part of your gas detection regime is knowing what to do before you enter an area, and what to do if the alarm sounds. Your crew needs to train for preparation and evacuation at regular intervals, for fast response under pressure. 

Most vessels will have their own routines, but some guidelines apply to everyone: 

  • Always measure an area’s atmosphere before you enter.
  • If an alarm sounds, exit to open air. 
  • Notify anyone in or near the affected area. 


We hope you found this guide helpful. Feel free to contact us if you have any questions regarding gas safety on board your vessels.


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