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Example COSHH Assessment - Step 2

Chemical Services > Health and Safety

Decide what precautions are needed

Aluminium powder clearly possesses several properties which make it a hazardous substance. Apparently, it will react with water to form hydrogen, a highly flammable gas; the dust can form explosive mixtures with air; and there is a limit placed on how much dust can be inhaled. On the other hand, the substance (aluminium) itself is not known to be an irritant to the skin or eyes, or be otherwise toxic.

The precautions in using the powder would appear to be fourfold:

To minimise the formation of dust clouds
To prevent the powder from being wetted with water
To keep sources of heat/flame/ignition away from any dust clouds
To minimise inhalation of the dust

Other precautions apply to the safe storage of the powder including identifying other substances that should not be stored alongside it (Sections 7 and 10 of the MSDS). In our scenario, the crime scene examiner will carry a stock of aluminium powder in his / her fingerprint kit, and the incident vehicle, so we should also concern ourselves with these additional precautions.

We are led to believe aluminium powder is hazardous, and we are given to understand the powder possess no risk to humans in terms of toxicity. On this basis, we could proceed directly to Step 3 of the COSHH Assessment.

STOP and THINK!

Always Question the Evidence

Should we rely entirely on the material safety data sheet for information? Is the information in the MSDS completely factual, up-to-date, and based on best evidence? Did we ask ourselves - what is the evidence? Did we ask questions such as:

What is the basis for the statement that aluminium powder will react with water to form hydrogen?
How much hydrogen is produced when aluminium powder is in contact with water?
If hydrogen is produced, under what conditions is hydrogen produced?
Will hydrogen be produced under the conditions used to develop fingerprints?
If clouds of dust are produced, what is the likelihood the cloud can be ignited or is explosive?
Is aluminium powder really non-toxic?

If not, our understanding of the hazards, and our assessment of the precautions needed to minimise risk to exposure is at least limited, and potentially inadequate.

Reaction with Water

Aluminium rapidly reacts with oxygen (in the air) to form a protective layer of aluminium oxide. The layer is formed almost instantaneously in ambient air and develops to between 2 to 5 nanometres (1 nanometre is 1 thousand millionth of a metre) thick, which prevents further reaction with the metal and oxygen. Over an extended time, this layer can become perceptible as a white, dusty film. In the absence of the oxide layer, aluminium will react with water to form aluminium oxide and hydrogen. In this reaction, aluminium is said to reduce water (to hydrogen) and in the process it becomes oxidised.



Since aluminium is typically coated with a layer of its oxide, it is said to be passivated, and it does not normally react with any water with which it has contact, and if any reaction does occur, it is extremely slow, and does not rapidly generate an appreciable amount of hydrogen. This might explain why aluminium window frames or car wheels or modern passenger jets or other solid, aluminium-containing items such as drinks cans do not pose a chemical health hazard! The passivated metal will also resist attack by dilute acids, dilute alkalis and concentrated nitric acid. The metal encountered in normal everyday life is therefore resistant to all manner of reactions, including that with water. Moreover, of all the metals, aluminium is second only to iron in its widespread, modern use.

Powders
Aluminium powders are typically comprised of micron-sized particles, and a small quantity of the powder has a huge surface area. For example, consider that a 1 gram sample of aluminium in the shape of a sphere would have:

a diameter of 0.891 cm
a volume of 0.37037 cubic centimetres
a surface area of 2.494 square centimetres

If that same 1 gram piece was then divided into smaller spheres each having a diameter of just 10 microns, each of those particles would have:

a volume of 5.236e-16 (0.0000000000000005236) cubic centimetres
a surface area of 3.1416e-10 (0.00000000031416) square centimetres


There would be 707 trillion (7.07xe14) particles and a total surface area of 222.2 thousand cm sq. That is an increase in surface area of almost 90,000 times!

It is this enormous surface area of powders that raises the concern about the possible reactivity of aluminium powder with water, especially at elevated temperatures, even though those used for fingerprint development are stabilised to oxidation and their reaction with water by use of organic/polymer coatings.

So, now we know a little about why aluminium powder will react with water to form hydrogen, and that the flammable gas is only produced rapidly when the surface is free from its oxide layer, and any other protective coating. Hydrogen is unlikely to be produced from stabilised aluminium powder at ambient temperatures and in normal conditions of dry storage and use, including the development of latent fingerprints, at least to any appreciable degree.

Effects on Health

Aluminium and its salts may enter the (human) system by way of direct contact (with skin and eyes), by ingestion (through the mouth: with food or in medicines or contaminated surfaces), or injection (cuts and injected medicines/vaccines) or inhalation (through the nose and mouth). Except for injection, only very small amounts of aluminum will enter the bloodstream through these routes.

Aluminium and its salts have long been implicated as toxic in all manner of human health disorders, perhaps most notably Alzheimer's disease (1), but the list is long. It includes
Blood - Microcytic anemia (decreased red blood cells or haemaglobin), Bones - Osteomalacia or aplastic bone disease (associated with painful spontaneous fractures, hypercalcemia, tumorous calcinosis), Brain/Central Nervous System - Alzheimer's Disease, Lou Gehrig's Disease (amyotrophic lateral sclerosis), Parkinson's disease, and Encephalopathy (stuttering, gait disturbance, myoclonic jerks, seizures, coma, abnormal EEG), Eyes - Conjunctivitis, local tissue destruction, Heart - Increased left ventricular mass and Decreased myocardial function, Lungs - Irritation of the upper respiratory system (via hydrolysis-liberated acid), Pulmonary fibrosis (reduced lung function), Muscles - Proximal myopathy (muscle weakness; muscle atrophy), Skin - Allergy, Eczema, and Dermatosis, and Other - Glucose intolerance, increased risk of infection. In short, aluminium can collect in blood serum, brain, muscle and bone, and is a suspected contributor to many disorders (2). Whether aluminium is carcinogenic (cancer forming) is not known, but aluminum has not been shown to cause cancer in animals (3).

Whether or not aluminium is actually responsible for many of these disorders is a question of real debate, and there is indeed some uncertainty in much of the peer-reviewed literature. The debate also extends to the plethora of other information on the wider Internet, some of which appears to be more scare mongering than evidence based. In the absence of definitive, factual, and conclusive evidence, the sensible approach must be one of caution (assume the worst) in deciding what precautions to take.

Powders
We note that the most likely routes for contact with aluminium powder are ingestion, inhalation and direct contact. Our assessment therefore must be restricted to these modes of contact and deciding what precautions should be taken to minimise such contact.



References:
(1) Sax, et al., Dangerous Properties of Industrial Materials,
Volume II, pp. 134-135
(2) http://www.osha.gov/SLTC/healthguidelines/aluminum/recognition.html
(3) http://www.atsdr.cdc.gov/tfacts22.html

Applying The Ideas

We find that the precautions listed at the top of this page are entirely appropriate.

Storage of bulk powder

Keep the powder dry in a sealed container
Keep the powder away from sources of ignition
Do not store the container with oxidising chemicals*

* Oxidising chemicals could include hydrogen peroxide and water, both of which are typically available to the crime scene examiner. Hydrogen peroxide, for example could be used as one of the reagents in the Kastle Meyer (KM) and Lueco Malachite Green (LMG) tests for blood.

Use of the powder

Minimise the formation of dust clouds
Do not overload the brush
Keep the powder dry
If wet, it won't work with the Squirrel brush anyway!
Keep sources of heat/flame/ignition away from dust clouds
Minimise inhalation of the dust

It's not rocket science!

Fire and Explosion

So now we turn our attention to the question of whether a cloud of aluminium dust can be ignited or is explosive?

The
temperature at which aluminium powders will ignite depends on the size and shape of the particles, the air to solids ratio, or dust concentration, the presence and type of any impurities, humidity, and the nature of any containment. The MSDS for aluminium gives this temperature at above 400 deg C. The Canadian Centre for Occupational Health and Safety (1), for example, gives these temperatures as:

For atomised (roughly spherical) particles:

  • 650 deg C for a dispersed cloud
  • 760 deg C for a layer


For aluminium flake (usually used in fingerprint development):

  • 610 deg C for a dispersed cloud
  • 320-326 deg C for a layer


These temperatures are certainly well above any encountered by our crime scene examiner in the ambient-temperature working environment, and the potential for a elevated-temperature-dependent dust-induced explosion can therefore be considered minimal.

At ambient temperatures, perhaps more than any other factor, possible
sources of ignition must be considered in the safe us of aluminium powders. Open flames, hot surfaces, electrical sparks and static discharges can all induce flash fires/explosions in clouds of dust from many types of substance. For example, even a cloud of flour in air can be made to ignite, given a suitable source of ignition.

Given a source of ignition, the
concentration of aluminium dust in air at which an explosion will occur (the minimum explosive concentration) has been reported (2) to be in excess of around 40 grams per cubic meter. The MSDS gives this value as 30 grams per cubic metre. We note that these values are both very much higher than the concentration of the powder that could be created by a crime scene examiner in dusting for latent fingerprints, and we may deduce that the potential for a concentration-dependent dust-induced explosion can be considered minimal.

As to
containment, if the dust cloud is unconfined, as would be the situation where our crime scene examiner is dusting for latent prints, the effect of any ignition, should there be one, is one of flash fire rather than explosion.

The aluminium particles are stabilised using an organic coating, so
humidity-influenced reactivity with moisture in the atmosphere is not regarded as a hazard. However, it should be appreciated that should a fire invloving aluminium dust or solids occur, the organic coatings may act as an additional fuel.

References:
(1) http://www.ccohs.ca/oshanswers/chemicals/chem_profiles/
aluminum_powder/working_alu.html
(2) "Standard for the Processing and Finishing of Aluminum,"
NFPA 65, National Fire Protection Association, 1987.

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