Technical Bulletin

Safer Chemicals - Real or Perceived?

We are all aware of the horrific health effects of radioactive materials and yet we have it in our homes.  Daily, we consume a highly toxic chemical which is commonly used as rat poison.   It's true - smoke detectors contain radioactive materials and our drinking water, as well as most toothpastes,  contains the highly toxic poison fluoride.   So why isn't the entire population disfigured or dead?

The simple answer is that the level of exposure to these harmful chemicals is the most relevant factor - not the actual toxicity of the chemical itself.

To simply state that one chemical is more or less safe than another and therefore less or more harmful is a statement without foundation.   As demonstrated in the above examples, our exposure to highly toxic chemicals is safe and not harmful at all if the levels of exposure are within safe exposure levels.  And it therefore follows that exposure to a "safer" chemical at these same very low levels would not make it more safe or less harmful.

Safe Levels - Exposure Standards

This leads us to the very important issue of quantification of "safe levels" of exposure to various chemicals in the work environment.  Most all chemicals have quantified Exposure Standards  usually expressed as TWA and STEL and are defined as:-

• TWA , (Time Weighted Average) is defined as the time-weighted average airborne concentration of a particular substance when calculated over a normal 8-hour work day, for a five-day working week. 

• STEL (Short Term Exposure Limit) is defined as the 15-minute time weighted average exposure which should not be exceeded at any time during a work day even if the eight- hour TWA average is within the TWA exposure standard. Exposures at the STEL should not be longer than 15 minutes and should not be repeated more than four times per day. There should be at least 60 minutes between successive exposures at the STEL.

• TWA and STEL are expressed in either ppm (parts per million) or mg/M³ (Milligrams per cubic metre).   The ppm and mg/M³ are interrelated according to the following formula

So what does all this mean and how does it relate to inkjet inks?   Let's look at some practical examples..

Inkjet Coders - Exhaust Concentration

The current day CIJ (Continuous Inkjet) inkjet coders consume, or exhaust to the atmosphere, an average of approximately 1 litre of solvent (make-up or additive) every 60 operational hours.  Many OEM's claim lesser solvent usage of up to 100+ hours per litre, but for this example we will use the worst case from a health & safety viewpoint.  This 1 litre every 60 hours equates to a discharge per operating hour of 16.6ml or 13.5 grams (16.6ml x 0.81 = 13.5gms -  since these solvents have a specific gravity of approx 0.81).  And where is this discharged to?   Into the air in the work area.

We now need to determine the "work area".  Practically speaking, the work area consists of the total volumetric area of the "enclosed area" in which the inkjet coder is operating.   If the open factory area (without airflow preventing walls)  is say 50 metres by 30 metres and 6 metres high, then this is the "work area" into which the solvent is exhausted.   Even without any mechanical ventilation systems, natural convection and air movement will disperse the solvent gasses over the full area.

But again, let's assume an absolute worst case scenario where the work area is confined to a very small 10 metres X 10 metres floor area and 5 metre ceiling.   A further very unlikely situation is assumed - that there is no natural or artificial ventilation and the air in this confined work area is never turned over for a full 8 hours - something that could only be the case if it were a fully sealed room and it's quite apparent that OH&S provisions would not allow this in any case.

Now we apply the following worst case calculations:-

• Work area volume in M³ = 10M X 10M X 5M = 500M³

• Solvent Discharge per hour = 13.5grams = 13,500milligrams

It's important to note that this 216mg/M³  is the accumulated maximum concentration at the end of 8 hours - not the actual concentration during the preceding 8 hours (refer to the TWA definition).   Again, we have used an unlikely example of a small confined work area with no doors, window or mechanical ventilation and have assumed no air circulation at all.  But we are highlighting an absolute worst case so we will use this figure.

Inkjet Solvents are All Safe

How does this concentration figure of 216mg/M³ relate to the Worksafe Australia's Exposure Standards for the commonly used inkjet ink solvents?   Following is a table indicating the TWA and STEL exposure standards for some of the most common inkjet ink solvents.

Table 1

It can be clearly seen from Table1 that even with this falsely high calculated concentration level, it is still well within the above TWA and STEL limits, even with the most hazardous Methanol and is less than half of the TWA of the most common solvent MEK.

But to labour a point, the calculated figure is not realistic.  Factories are larger and open, have natural or mechanical air flows or if they are smaller contained rooms, will have mechanical ventilation systems.  The concentration calculation is at the end of the 8 hours - not for the full 8 hours as provided for in the TWA limits.  In short, the concentration levels calculated above are never likely to be achieved, indeed, levels that are a fraction of this figure are most common in actual situations.

To highlight this, an air quality test was performed in the Heinz factory at Dandenong Victoria several years ago. It was initiated after operators near the inkjet coders were complaining of strong solvent smells.  Despite the strong smell of solvent, the tests showed that maximum  concentrations were in the region of only 4 - 5mg/M³  around the operators and up to a peak of 12mg/M³ directly adjacent to the air/solvent outlet of the inkjet coder.

The results of this air quality test highlights two significant facts.   Firstly, the solvent vapours dissipate well in the presence of small, natural air moments and are only present in very small values.  The second point is that of odour threshold.

Odour Thresholds - They Can be Misleading

Our sense of smell is an excellent detector of potential danger - or is it?   The natural, but often wrong assumption is that the stronger the smell the greater the volume of gasses and hence greater hazard.   This is certainly not always the case.

Many highly dangerous chemicals and gasses have no odour at all.  Carbon Dioxide is a perfect example - it has no odour at all and can be at deadly concentrations without being detected by smell.   Another example of a dangerous chemical is LPG which has no detectable odour at all in its natural state.  To provide an alert mechanism, a highly pungent compound (ethyl mercaptan) is added in the refining process to ensure we can smell LPG if it escapes to air.  A common inkjet ink solvent, Methanol (Note 1 below), is virtually odourless and could achieve high concentrations without detection.

The converse also applies where extremely small volumes of a chemical will result in our incorrect perception, through smell, of very high concentrations.  These chemicals have what is termed a low odour threshold and an example of this was just quoted above, ethyl mercaptan, which is in LPG in minor volumes but produces a very strong and apparent odour.   Perfumes and scents are classic examples of compounds that have high odour in minute volumes - they have a low odour threshold.

It's fair to say that it's common for low odour threshold chemicals to be wrongly perceived as more hazardous in the workplace whereas in fact they can be less hazardous.  Because of their more intense odour when smaller amounts of the chemical are present, they provide strong sensory warnings well before any TWA or STEL levels are reached.   Hence the most common inkjet ink solvent MEK, which has a very low odour threshold and a distinctive smell, is very apparent to workers at levels well below or even at a fraction of the TWA or STEL concentrations. 

Safer? - It's Either Safe or Un-safe!

So the final question is "Which of the inkjet ink chemicals is safer?"   

It may sound ridiculous, but this question is similar to a question like "is it safer to drink MEK or methanol?"  It's very obvious that both are going to have a severe effect and neither one is safer than the other if you drink them.   Put both these chemicals in sealed containers on the desk in front of you and then ask "which is safer now they are both in sealed containers and cannot touch me?".   It's equally as obvious that neither are harmful at all if they are contained and we're not exposed to them.   So again in this scenario, one is neither safer nor more harmful than the other.   Even though the TWA exposure standard of MEK (445mg/M³) is nearly twice that of methanol (262mg/M³), it has no relevance as far as more or less safe in either of these two situations.

Whilst exaggerated, the important point made here is that the harmful effects of chemicals are purely relative to exposure levels - just like the radioactive smoke detectors and fluoride water and toothpaste examples.  But more importantly, it clearly demonstrates that in extremes of exposure concentrations, neither chemical can be considered more or less safe than the other.   The only time "more or less safe" criteria can possibly be applied is at concentration levels in between the TWA's of the two chemicals being compared as shown in Table 2 below. 

Table 2

But this Table demonstrates that the "more or less safe" assessment argument has a single major flaw.   

From the above Table 2, it's clear that with workplace concentrations below 262mg/M³, (the TWA of methanol), the chemical is safe to use - as is MEK.  If the concentration levels exceeded 445mg/M³, (the TWA of MEK), neither MEK nor methanol would be safe to use.

However, with workplace concentrations between 262mg/M³ and 445mg/M³, methanol is above its TWA and is not safe to use whereas MEK can be used since its TWA is not exceeded.  So does this indicate that MEK is "more safe" than methanol in this situation?   The answer to this highlights the flaw in any "more or less safe" argument.

If the actual workplace concentrations were between 262mg/M³ and 445mg/M³, then it's true - MEK may be cited as "more safe" to use than methanol.  But the flaw in this assessment?   It assumes methanol is being used in an un-safe environment (above TWA's) which should never happen.  MEK is not "more safe" here, it's simply that MEK is within TWA's and deemed safe whilst methanol is above TWA's and deemed un-safe.

In short - there is only workplace concentrations of various chemicals that are deemed either safe or un-safe - there is no such thing as "more or less safe".

Conclusions

 The solvents used for the more common inkjet inks are all classified as hazardous.   But as can be seen from the above, the concentration levels of these solvents in the application of inkjet printing are commonly very low.   The workplace concentration levels are but a fraction of the TWA's for even the lowest rated solvents and therefore they are all equally as safe - none are "more or less safe".  

Note1.

Some OEM's state that their Methanol based inks are "Alcohol" based and subsequently stating or implying that "alcohols" are safer than other ink solvents. One example is the Marconi 16-8700 ink which is listed as an alcohol base ink.   Whilst it is correct that Methanol is in the chemical family of alcohols, it does however have significantly different health effects and safety concerns than other alcohols.  It had a systematic effect, is easily absorbed through the skin and causes irreversible damage to the optic nerve (blindness).   Methanol is considered and classified as a poison under the Poisons Schedule Number S6.   Beware of Methanol inks being called "alcohol" inks and take the necessary precautions in handling them.

Footnote

Inkjet Solutions Pty Ltd is in a position to comment with authority on this subject.   As a manufacturer of inkjet inks, the company management and technical management (qualified chemists), have a responsibility to make sure that the manufacturing workplace is a safe working environment for staff.   To ensure this, a comprehensive study and understanding of TWA's,  STEL's, concentration levels, odour thresholds and health hazards of all the chemicals used in inkjet inks is an integral undertaking of Inkjet Solutions Pty Ltd management.  

© Inkjet Solutions Pty Ltd 2002