Air quality

Published: 10th May 2010

Occupational or Industrial Hygiene has its roots in the early 1900s and is defined as the RECOGNITION, EVALUATION and CONTROL of those factors in the workplace that may give rise to sickness and impairment of the workforce.

A Review of Industrial/ Occupational Hygiene

The RECOGNITION on the face of it should not be that difficult as over 2000 years ago Hippocrates recognised a danger from asbestos and Pliny the Elder recognised the dangers of mercury exposure. Over the following years, many scholars and scientists have come to recognise effects and affects of toxic and pollutant materials.

Recognition however covers many issues and it is the degree of those issues that affect the way we can and have to identify the cause.  For example it is no good monitoring over hours such things as Carbon Monoxide, Hydrogen Sulphide and Hydrogen Cyanide when relatively low levels (0.1%) will kill quite quickly.  In these cases we need to identify, qualify and alarm in a very short space of time.

So recognition, is it a chemical, is it a gas or vapour or dust or particulate or fibre?  Is it physical, i.e. light, heat, noise, humidity etc., could it be biological and what about the man-machine issues of R.S.I. etc..

The likelihood is that it may well be a combination of several and so Risk Assessments are needed to evaluate the potential for harm followed by some sampling/monitoring programme.

Why do we want to sample the air?  For several reasons:-  Health Protection, Environmental Protection, Compliance with/to Government legislation, Process Protection (leaks cost money) and Compensation Claims.

So what are the basic questions we need to ask?  Who, what (recognition), when, where and of course how, are the base questions.

Just quickly going back to our Recognition, Evaluation and Control definition, we have identified the recognition side, the Evaluation is the potential for harm and the Control is the way (or ways) we may go about resolving or improving the situation. Things such as substance elimination from the process – though this is probably very difficult to do without major changes in the product and process.

Substitution of a harmful product by a less harmful one has and is used with some product change but with the objective of lowering air exposure concentrations to the higher toxic materials.

Isolating the worst parts of the process may help in reducing the number of people exposed and assist in containment and treatment.

Reduce the exposure time by job sharing again may help in lowering individuals personal exposure levels.

The use of Personal Protective Equipment such as gloves, masks etc. can significantly help if careful selection and use is made of the equipment.

Before we wander too far away, let us revisit our Recognition section, we have identified the main streams of problem and now need to look in more detail to the chemical properties and nature of those things we may be exposed to.

Again more questions and answers:-

Is the material flammable – if so there is the potential for an explosive atmosphere and so careful monitoring with fast alarms is essential?  There are many manufacturers of these types of equipment which are well known and well established.

Another of prime importance is Oxygen – or the lack of it arising from entry into confined spaces such as tanks, underground rooms and holes.  It only takes a few % lowering in the oxygen concentration for impairment, unconsciousness and death to occur.  Again immediate monitoring with alarm is imperative and many companies produce instruments specifically for this purpose.

Other factors such as extreme toxicity and carcinogenicity are very important in deciding the best course and implementation of monitoring techniques.

Odour and allergy effects are also important as odour can be very emotive but in a lot of cases not too damaging as our sense of smell may detect very low levels that are not indicative of harmful effects and well below any set Threshold Limit Value (TLV).

When it comes to particulates, there is a need to know and recognise the particle size as this is important in evaluating the harm to health as defined by the appropriate TLV.  Particulates will be recognised as either Respirable – small particles below 16 micron or Inhalable – larger particles up to 100 micron.

The need to know will determine which is the most appropriate sampling equipment and conditions to use – vital to ensure correct results and interpretation.

The likelihood is that we will be looking at mixtures of several of these factors and so must take a risk assessment to assess the primary requirement of life/death or health action.

The other effect is the effect and affects of skin absorption as many chemicals are known to produce harmful health effects via the skin.  Dermatitis being the number one industrial problem.

There are many different monitoring techniques, some for quick assessment such as colour tubes and others for immediate and longer term monitoring with associated alarms including:-   Paper Tape instruments, Portable Gas Chromatographs, I-R and U-V instruments, electrochemical and semi-conductor sensors and Flame/Photo ionisation devices.

For odours, we usually need to take an air sample or ‘grab sample’ as it is known, using canisters, glass bottles or more usually ‘Gas Bags’.  The latter are tried and tested bags made from inert plastic film that can be filled very quickly and transported to a laboratory for investigative analysis.

Whatever is used must be ‘fit for purpose’ – reliable and precise, operate under the working conditions, be as small and lightweight as possible, easy to sue and fast to respond.  Also required is instant warning where required or ease of analysis when sampled with possible automated analysis.

On top of all this we have to factor in those conditions beyond our control, the environment of wind, temperature, humidity, pressure and movement – just for starters.

Let us take a look at Gas and Vapour sampling in a little more detail:-

Benzene was the first chemical trapped on a sorbent material back in 1935, in 1936 a weighing procedure was used – not very effectively – to measure the mass of trapped pollutant in a sorbet.  It was in the 1970’s that charcoal tubes were being successfully used to trap VOC’s with subsequent Gas Chromatography analysis.  However, the U.S. Dept. Of Health is on record as saying this technique is novel but will never catch on so not worth a patent!

Personal samples must be taken within the breathing zone which is currently defined as being a 30 cms hemisphere around the nose and mouth.  The sampler is usually positioned on the worker’s lapel with other equipment attached to the worker’s belt.  New work is looking at positioning the sampler closer to the breathing zone using flexible tubing allowing the sampler inlet to be within 5 cms of the breathing zone.

Sorbents, commonly used for gases and vapours need to have many properties – they should be chemically inert, have low affinity for water and carbon dioxide, must not form dust and have a high capacity at ambient temperature for the pollutant under study.

A typical make up of a sorbent tube is shown in the diagram.

The back section is there as a quality check to ensure good sampling.

When using sorbent tubes in any application they must always be VERTICAL and the correct way round with the back-up section towards the sample pump.

Another widely used technique for gases and vapours is DIFFUSIVE or PASSIVE samplers.  The devices have no pump and rely on air movement by diffusion.  Developed and been commercially available since the 1970’s they are used in both monitoring and sampling strategies being able to detect down to the P.P.B. (parts per billion) levels.

There is a very wide variety of sorbent tubes now available, coconut charcoal is still the most popular and is used for a wide selection of V.O.C’s.  Other tubes are for specific compounds, such as Formaldehyde, and these can be coated or treated with selective chemicals.  Some of these tubes have defined shelf life (storage time) and storage conditions often being required to be stored at refrigerated temperatures.  It is very important to ensure that you have the correct tube for your particular chemical of interest otherwise you may not get a sensible result and even no result at all.  Many tubes are made in glass with sealed ends, these must be opened before use and in many cases require a solvent to be added to release the trapped pollutant before analysis.  Other tubes made in metal can be heated to release the trapped pollutant (thermal desorption).

It is important at all times to seek advice from the supplier and the analytical laboratory before venturing on a sample programme to ensure the correct product is being used – this will save both time and money!

Let’s move on to Particulates – particulates as have been mentioned are generally measured as either Inhalable (everything that is breathed in) or Respirable the smaller sized particles that enter deep into the lungs.  A study of dust in mines was conducted back in the 1570’s by Agricola, Louis Pasteur hit upon Bioaerosols and Respirable sampling really started in the 1950’s.

Fumes and smokes (including tobacco smoke) exist as small particles which come under the Respirable fraction and are cause for concern in lung diseases.

In the past the expression “Total Dust” was used before the term “Inhalable Dust”.  This total dust was seen to reflect the total amount that the sampler would sample over a given time at a given flow rate.  However, it is not a true measure of the true concentration as the samples used have been shown to grossly undersample many pollutants and indeed sample at differing efficiencies.  It was this that lead to the International standards of definition being agreed by parties in the U.S.A., U.K. and E.U. being agreed and accepted.

Inhalable and Respirable dust fractions are sampled using differing devices and flow rates.

There are several “Inhalable” samplers on the market but the most commonly used and specifically designed sampler is the I.O.M. (Institute of Occupational Medicine).  Developed and designed at the I.O.M. in Edinburgh Scotland, this sampler uses a cassette system with filter to collect the inhalable particles and the whole cassette plus filter is pre and post weighed (gravimetric analysis).  This sampler operates at 2 litres/minute.  Other samples such as the conical inhalable sampler, button sampler, 7 hole sampler are used but be warned – check the required flow rate as they do differ and it is imperative to use the manufacturers recommended flow rate to ensure sample validity.

Now to Respirable, these much smaller particles only up to around 16 micron that get deep in the lungs.  Since the 1950’s a device known as the cyclone has been used.  This device separates incoming particles by spinning and large particles fall out into a grit pot with the smaller particles being collected on a filter paper inside a cassette.  This time however, we ONLY WEIGH THE FILTER before and after sampling.  The current U.K. cyclone designs operate at 2.2 litres/minute, but other devices are available but may well operate at a different flow rate – check with the manufacturer.

It has been possible to sample BOTH Inhalable and Respirable using polyurethane foams in the I.O.M. sampler.  This uses the properties of the foam with a specific pore size and density to size select the different fractions.  It is the only way to get both results from one sampler – you cannot do this using the cyclone.

Dusts and particles can also be monitored using hand held devices.  These devices use techniques such as light scattering to enable the user to see exactly what the concentration is at any given point and time and are really useful for surveys.  They can store data which can be down loaded to a P.C. showing profiles and overall concentration.  Similar devices also are available for general and/or specific gas and vapour sampling as well.

One of the critical factors relating to sampling is ensuring a known rate flow through the sampling device.  So a selection of flow calibrators ranging from soap film meters to rotameters to electronic displacement units are available depending on the accuracy required and the budget available.  Flow rates should be measured both before and after sampling to ensure a stability of +/- 5% and a Primary Standard should be used – in this case and many others requiring the use of the electronic displacement unit.

The choice of the sample pump is also important due to safety requirements (Intrinsic Safety and CE) and flow.  Ideally the pump should be capable of a wide flow range from say 50 millilitres/minute up to 4 litres per minute to cover all applications.

As we have said, gases and vapours are usually sampled at low flows (50-200 mls/min) and dusts and fibres at 2 litres/min.

This latter flow is similar for Bio-aerosols, which includes a variety of different sources and subsequently a variety of different sampling methods and techniques.  Some using the same equipment as for dusts, others being specialist pieces of equipment.  The object in bio-aerosol sampling is to keep the micro organisms alive so any direct impact device is probably not the best.

Another area of concern is the Indoor Environment, the office or any other area not considered to be part of the manufacturing process.  Several volatile compounds from floor coverings, worktops, decorating can be considered harmful and certainly can produce headaches, eye soreness and a feeling of general sickness.

The opposite of Indoor is the Outdoor Environment where typically different types of chemicals are used, such as pesticides and herbicides.  This environment usually requires much larger volume samples being taken as the airborne concentrations are lower.  This requires sturdier, heavier equipment for sampling combined with more sensitive analytical techniques possibly all integrated with meteorological equipment for wind speeds and direction.  Small particles such as PM10, PM10, PM2.5 and PM1 can be measured as daily pollutants from traffic for example.

There is also the sampling of exhaust from stacks and ducts and landfill, all requiring sensitive equipment often with remote wireless communication via P.C.s to control and monitoring stations.

Related to most of the above is the contamination of chemicals on or via the skin, from direct contact or via surfaces from spills or airborne deposit.  There are several techniques that give instant warning of heavy metal contamination on surfaces and other analytical techniques are also available.  Many chemicals are harmful by skin absorption and especially the transfer of contamination via tools and door handles from work areas to office areas can lead to problems.  Various decontamination fluids are available and surfaces and equipment should be wiped down periodically.  So what about the rules and regulations and the whys and wherefores?

All instrumentation and most methodologies are covered by European or International guidelines.  Any item of equipment or sampling technique has certain criteria and standards that should be met.  For example, equipment must be covered by a CE mark for compliance to the relevant criteria for the purpose of use – in our case it is for electrical and magnetic signal receiving and transmitting.

Also now we have the Waste Electrical Equipment regulations, the Battery Directive, Intrinsic Safety and Restriction of Hazardous Substances.  All of these require design, development, test and of course the associated cost.  For example, just to pass the Intrinsic Safety Standard (Atex or IECEx) will involve test costs of several thousands of pounds per device.

Other International Standards that are worth noting are:-

EN481          Workplace Atmospheres – size fraction definitions

EN1232        Pumps for Personal Sampling

EN1076        Sorbent tubes for gases and vapours

ISO16000-4  Formaldehyde by diffusive sampling.

As with the Intrinsic Safety requirements, there is now more resposibility on employers to identify the risks of the chemicals and areas where they are used to ensure their employees and others are using safe equipment in safe areas with safe chemicals.

It can be seen from all that has been written that there is quite a bit to do when looking at sampling techniques and equipment and one may be forgiven for thinking it is just too complicated.  However, the taking of samples and the monitoring of the individual and the environment has huge and significant benefits.

The main thing is to be consistent, repeat in a truly reproducible manner – this will give the best and most accurate results.

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Published: 10th May 2010 in Health and Safety Middle East