Measuring benzene the right way

What is benzene?

Benzene is a chemical substance that is a colourless or light yellow liquid at room temperature. It has a sweet odour and is highly flammable. However, far more important than the flammability of this gas is the fact that it is extremely toxic and also carcinogenic. Benzene evaporates into the air very quickly. The vapour is heavier than air and can sink into low-lying areas.

Benzene dissolves only slightly in water and floats on top of the water.

Where does benzene occur?

Benzene is formed both by natural processes and by human activities, largely in the pharmaceutical or chemical industry. Natural sources of benzene include volcanoes and forest fires. Benzene is also a natural component of crude oil, petrol and cigarette smoke.

Benzene is one of the top 3 most important raw materials for the chemical industry, alongside ammonia and ethylene. The Netherlands is one of the world's leading benzene producers. Some industries use benzene to make other chemicals that are used to produce plastics, resins, nylon and synthetic fibres.
Benzene is also used to make some types of lubricants, rubbers, dyes, detergents, medicines and pesticides.

Outdoor air contains low levels of benzene from tobacco smoke, petrol stations, motor vehicle exhaust fumes and industrial emissions. Indoor air generally contains a higher benzene content than outdoor air. The benzene in indoor air comes from products that contain benzene, such as glue, paint, furniture wax and detergents.

Benzene can also be released during work in the ground. During soil remediation, for example, where benzene vapour is present in the contaminated soil. It is released during the work and employees can be exposed to it.

Benzene can also be found in oil or oil derivatives. In the oil and gas sector these are safely transported and stored in process pipes and storage vessels during normal operation of the installation. During routine maintenance of the installation, or for instance as a result of faulty flanges, employees may potentially be exposed to benzene. Measuring benzene is a first line of defence for tracing sources and preventing exposure.

The hazards of benzene

Benzene is readily absorbed via the airways, the skin and the mouth, the airways being the main route. Once absorbed, benzene is distributed throughout the body.

''Benzene vapour irritates the airways; benzene is also irritating to the skin. Shortly after inhaling very high concentrations of benzene, people may develop symptoms indicating toxicity of the central nervous system, such as dizziness, convulsions, tremors and ultimately a narcotic effect and death from respiratory arrest.'' According to the Dutch Health Council

Direct consequences of inhaling high levels of benzene can include:

Drowsiness or sleepiness
Dizziness
Rapid heartbeat
Headache
Confusion
Pale skin
Bumps on the skin
Tightness in the chest
Unconsciousness
Euphoria
Weakness
Death

In practice, however, it is often a matter of inhaling small quantities over a longer period. Occupational exposure has long been associated with adverse effects on the blood and bone marrow, including a shortage of both red and white blood cells. Various studies of workers exposed to benzene demonstrate these effects at high but also at relatively low exposure concentrations.

An increased risk of leukaemia following exposure to benzene has been found in various occupational groups and in different branches of industry.

Exposure limits for benzene 

On the basis of these data, the Dutch Health Council applies a health-based recommended value of 0,7 mg/m3 (0,2 ppm). This has also been introduced nationwide as of 1 October 2017. 

Such a low limit value therefore also demands a low resolution from the measuring instrumentation.

How should I measure benzene?

Solutions widely used in the occupational health and safety sector are: benzene-specific PID meters, analysis of gas samples, or detector tubes. The difference between these three is that with a PID meter you can actually follow gas concentrations in real time. With the other two you only get a snapshot.

In the field a PID, or Photo Ionisation Detector, is most commonly used. This sensor detects hundreds of volatile organic compounds (which therefore includes benzene). When your PID shows anything other than zero, you know that volatile organic compounds are present. However, the PID is not able to identify which substance is actually being measured. 

There is of course a solution for this: the benzene-specific separation tube. These are small tubes developed specifically to let only benzene through and to hold back all other gases. That way you can scan the work site for volatile organic compounds without a benzene tube. As long as the measuring instrument shows zero, the coast is clear*. Then, when your PID meter does register a reading, you can fit a separation tube. If the value drops back to zero, you know it was a different gas, which is now being captured by the tube. If the value on your screen remains unchanged, you are dealing with benzene. 

As a rule, standard PID meters have a resolution of 1 ppm, which in this case is already five times higher than the limit value. Not a very effective solution, then. For benzene and its low limit value, specific instruments are therefore developed that can do this. Below you can see two examples.

*Please note: there are exceptions in which negative cross-sensitivity affects your measurement results. Always keep thinking for yourself and seek advice from a specialist if necessary.