When it comes to the control of exposure to hazardous chemical substances, the Material Safety Data Sheet or MSDS provides more value than just being an item on a checklist. It is a vital document that gives us a few key pieces of information regarding the “personality” of the chemical or mixture which we can use to design our engineering control systems at the workplace.
The Ideal Storage Environment
As one of the more obvious characteristics, the “Flash point” of a chemical tells us if it is flammable, sometimes also called inflammable, and it’s measured as the minimum temperature to which the vapours released by the chemical inside its storage container need to be exposed in order to ignite. It’s called the “Closed Cup Flash Point” and is a scientific test method used to get to that temperature.
The lid is sealed and the ignition source is introduced into the vessel itself, allowing for a closer approximation to real-life conditions (such as those found inside a fuel tank).
The four main kinds of closed cup flash points are Pensky Martens, Abel, Tag, and the most widely-used, Small Scale, which is often known as Setaflash. However, new developments in flash point testing have led to the introduction of modified safety standards. Once such standard is the ASTM D7094 – Modified Continuously Closed Cup Flash Point Standard Accepted as a Safe Alternative Method in Various Fuel Specs.
Flammable chemicals or liquids require special storage rooms or cabinets inside the workplace to prevent explosion and fires during storage. It also requires to be used in Smoke Free zones, which is rather common in the workplace these days.
Some countries also impose limitations on the maximum amounts one can store on site, which if exceeded may require special licenses or permits.
This characteristic is also important as it allows us to make informed decisions in the design of storage facilities and the layout thereof. Chemicals may be non-flammable or non-toxic, but when mixed with another chemical, may cause violent reactions which could release heat or toxic vapours.
The pH of a chemical also allows us to provide separations in our storage design, particularly in respect of automated fire protection systems. Corrosive substances may, for example, react to water as extinguishing medium and should be kept separate from flammable substances. This will eliminate the need for various types of fire fighting appliances and the accidental use of the wrong medium.
pH is also an indicator of the potential of a corrosive substance to cause damage to its own storage container over long periods, giving us it’s possible “storage life” for one, while also allowing us to select the correct surface coatings for storage facilities to prevent long term damage.
The pressure exerted by a gas or vapour in equilibrium with a solid or liquid in a closed container at a given temperature is called the vapour pressure. Where VP is indicated on the MSDS at STP – Standard Temperature and Pressure as defined by IUPAC (International Union of Pure and Applied Chemistry) as air at 0 oC (273.15 K, 32 oF) and 105 pascals (1 bar).
For example, Acetone as a Vapour pressure of 24.7 kPa or 0.247 bar.
As our planet does not have an ambient temperature of 0 oC, the SATP is mostly used or the pressure is indicated at a specific temperature.
In other terms, the vapour pressure is the force exerted on the walls of a closed container by the vaporized portion of a liquid. If a container then has a design pressure rating of 1 bar, it would be safe to use it for acetone.
Vapour pressure increases with an increase in temperature. That is why most aerosol spray cans indicate a maximum storage temperature or a warning that it may burst if exposed to a higher temperature than stated.
Liquefied Petroleum Gas (LPG) however does not naturally (as PG) have a vapour pressure as it is too low to be in a liquid state at standard atmospheric pressure. Pressure is added mechanically, to turn the gas into a liquid in order to store and transport it. In its liquefied form it has a vapour pressure of 637 kPa @ 27°C (Note, not at STP)
The Ideal Use Environment
The use of chemicals at work expose us to a variety of dangers. When it comes to reduction of these hazards, we have one objective. To remove the danger to humans and to property under normal operating conditions. These are the conditions we can control. Failure state conditions or emergency conditions may require additional measures, but we won’t cover those in this article.
The vapour density of a substance is an indication of where an accumulation of released vapour is most likely to occur. Where a substance releases a vapour that is heavier than air (=1), the lowest section of the workplace is where it will accumulate. Vice versa, vapours will rise and build up inside corners and cavities of roofs and ceilings.
This is an important aspect when it comes to the design of ventilation and extraction systems. It would be pointless to rely on open windows and natural ventilation where a substance like Acetone with a Vapour Density of 2 is present. The vapours could build up and displace the breathable air, creating confined spaces and causing fatalities.
Vapour Density is therefore the relative weight of a gas or vapour compared to air, which has an arbitrary value of one. If a gas has a vapour density of less than one it will generally rise in air. If the vapour density is greater than one the gas will generally sink in air.
In Gasses, we also refer to the Specific Gravity where air = 1
Note: The Vapour density of a substance is not the same as the Relative Density of that substance. See further.
The Vapour density is also important when it comes to the positioning of luminaires and other electrical apparatus, although generally, the installation of intrinsically safe electrical apparatus are required where the Explosive Limit Range of a substance is substantially big enough, e.g. 2,5 to 95%. See Classification of Hazardous Atmospheres.
A mixture of a flammable vapour or gas and oxygen causes ignition. Think of a motor vehicle with a normally aspirated engine. If you step on the gas pedal when the vehicle is starting, the supply of fuel exceeds the upper explosive limit of the mixture and the vehicle is said to “flood”. This same principle is applied where a vehicle is fitted with a “choke”. It increases the ratio of fuel to air during a cold start. As the operating temperature increases the ratio is gradually reduced (less fuel for the same amount of air).
The MSDS should provide you with two values as a percentage. In the case of acetone it is 2.15 to 13.3%
In the case of Acetylene, it is 2.5 to 82%
Above or below these values, the risk of ignition or explosion is minimal.
This value is important for two reasons. The first is to determine the number of air changes (ACPH) a ventilation system must be able to produce relative to the concentration of the gas or vapour. The second is the identification of areas where and when a Permit to Work may be needed.
This section of the MSDS deals specifically with the exposure of Humans to a substance. It is an indication of how much of a substance is needed to kill a specific population or specie indicated as the LD50 indicated as mg/kg.
The LD50 is then used by Occupational Hygienists to determine the maximum safe levels of exposure to that substance and this is indicated, often in a regulatory form, as the OEL or TLV or threshold limit value. These values are then in turn split into a STEL (Short term exposure limit) and TWA (Time weighted Average) limits.
The above values are related to airborne concentrations and are either expressed as parts per million (ppm) or milligrams per cubic meter (mg/m³).
The LD50 is not reached when the OEL/TLV is reached as the latter is a much lower value, thereby creating a margin of reasonable safety, but as each individual has different physical straights, there is no 100% certainty that exposure within these values will not cause adverse health effects. The best solution would always be to consider alternative substances, but as a producer, this is obviously not the solution.
Acetone once again as an LD50 of 9750mg/kg and is indicated as “oral rat”. This means that scientific tests were carried out on rats by giving them increasing oral doses of acetone until 50% of the test population dies. (Not good news for Lab Rat Lawyers)
For humans it means that a person with a weight of 80kg, would need to ingest (80 x 9.75) 780g of acetone >99% pure to stand a 50% chance of being fitted with a wooden suit. Fortunately acetone is often just an active ingredient in most popular every day substances.
Accumulation of a chemical in the human body is indicated by a BEI or Biological Exposure Indices. When a person exceeds the BEI, he or she must be removed from the workplace as result of over-exposure and may only return to work on advice of an Occupational Medicine Practitioner.
Toxicity also has a variety of indicators, but in most cases it refers to acute toxicity.
Various institutions have different OEL values, depending on how risk averse their standard generating committees were at the time.
In order to minimise the OEL / TLV, a ventilation system should provide sufficient ACPH to reduce the concentration below these values.
The environmental controls needed for chemical substances can be guided by looking at the MSDS section on Ecological information. This indicates the toxicity of a substance to aquatic life and also have a set of parameters to look at.