Hazard and Risk

HAZARD AND RISKS IN ORGANIC CHEMISTRY

The difference between hazard and risk

1 Many organic substances, including those you might come across in the school Jab, require

special handling because they are hazardous and carry a risk of harm to people using them.

2 In everyday conversation the words hazard and risk may be taken to mean much the same

thing — that something is potentially dangerous. However, in chemistry these words have

very specific meanings:

3 Hazard — the hazard presented by a substance or an activity is its potential to do harm.

This potential is absolute. For example, some chemicals are flammable and some are toxic.

Their tendency to burn, or to poison, are tested and calculated and will always be the same. –

4 Risk - the risk associated with a particular hazard is the chance that it will actually cause

harm.

(a) Risk is affected by a number of things, in particular the nature of the hazard involved

and the level of exposure to it. The level of exposure in turn is dependent on factors

such as the expertise of the person working with the chemical, the volumes being

used, the conditions in which it is used and the protective clothing and equipment

available.

(b) A hazardous substance can be safe to use if the risks are minimised. For example,

you know that petrol is hazardous — it is flammable and poisonous. However, the risk

of coming to harm when you fill a car with petrol is very small. On the other hand, if

you spill petrol on your clothing and then light a match, the risk of harm is high. The

hazard presented by petrol is the same however it is used — but the risks of harm

changes considerably depending on how the petrol is handled.

5 How can you reduce the risk associated with using hazardous chemicals? Some strategies

are listed below:

(a) Working on a smaller scale — for example, there is less risk of inhaling fumes with

smaller amounts of chemicals because there are fewer fumes. ‘When the quantities

are small it is easier to contain the reaction in closed apparatus, and to carry out

the reaction in a fume cupboard. If heat is given out (exothermic reaction) this will be

less for smaller quantities of reactants, and smaller quantities are easier to transfer

from one container to another without spillage.

(b) Taking specific precautions or using alternative techniques depending on the

properties of the hazardous substances you are using — for example, one way of

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reducing risk is to use the lowest possible concentration of a solution to achieve a

particular reaction. Using. a low concentration considerably reduces the risks for a

hazardous solute. The lowest suitable concentration of any particular reagent will vary

from reaction to reaction, eg in many laboratory reactions 0.4 mol dm -3 or even 0.1 mol

dm-3 acids and alkalis are quite adequate. At these concentrations the chemicals are

irritant rather than corrosive, so the hazard level is reduced. This in turn reduces

the level of risk. However, there are other reactions, particularly at AS and A2 level,

that require 1 mol dm-3 and even 5 mol dm-3 solutions to be used. Although the hazard

level is higher, the risk can be kept low by careful planning and risk assessment.

(c) Careful use of safety measures — such as fume cupboards to remove toxic or

flammable fumes, and personal protection such as safety goggles, considerably

reduce the risk of carrying out reactions using hazardous chemicals.

(d) Changing the conditions under which a reaction takes place – for instance, lowering

the temperature of a reaction mixture will slow the reaction down. This can

substantially reduce the risks of the reaction mixture overheating and/or excessive

fumes being given off, though cooling may change the equilibrium position and so

affect the proportion of reactants and products in the final mixture. For example, the

reaction between sodium thiosulfate and dilute hydrochloric acid is often used to show

the effect of increasing temperature on the rate of a reaction. However, the sulfur(IV)

oxide, SO2 given off can trigger asthma attacks in vulnerable individuals. By carrying

out the experiment in closed vials and reducing the temperature immediately after

readings have been taken, the amount of gas given off can be greatly reduced and

almost all students can carry out the investigation without suffering any ill effects.

(e) Using alternative methods with less hazardous substances — sometimes it is

possible to substitute chemicals and still study the same basic reaction. In many cases

the alternative chemicals are not as effective as the original, more hazardous ones.

The reaction may be slower or the yield may be lower but if the risk is also

substantially lower these disadvantages are worthwhile. CLEAPPS (the Consortium of

Local Education Authorities for the Provision of Science Services) is an advisory

service providing guidance on practical science teaching in schools and colleges. They

have produced a list of alternative compounds which can be used in organic chemistry.

For example, tetrachloromethane was the best solvent to make a non-aqueous

bromine solution because the solution would keep for a long time. However,

tetrachloromethane has a high hazard rating (it is toxic). Cyclohexane can be

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used instead if it can be stored out of the light (eg in a dark bottle or in a clear bottle in

a labelled cardboard box). However, plastic screw tops or bungs will be affected if it is

stored for over a month. In a clear bottle, noticeable deterioration will occur within two

days. So there is often a safer alternative, even though it may not perform quite as

well.

Wearing a mask while spraying helps to avoid any risk of inhaling the chemical.

However, the risks of starvation and malaria are far higher than any potential risks

from pyrethrin sprays. On the other hand, there are other much more effective

pesticides that can kill many more pests and so save many more people from

starvation.

(e) Synthetic pesticides can be very effective at destroying pests — often organic

compounds, they help prevent the destruction of millions of tonnes of food every year.

However, they can be expensive, and there are some significant risks associated

with their use. The chemicals are often very persistent in the environment. They are

not broken down readily in biological systems and so they build up in food chains

until they reach a level where they are harmful to humans or other animals. They can

also be toxic to the farmers who work with them. This is a particular problem in the

poorer countries of the world, where pesticides are often still applied by hand. In

China, for example, up to 500 farmers die each year from acute pesticide poisoning.

Careful use of the chemicals, wearing protective clothing and breathing equipment —

even a simple mask — can reduce these risks. If several farmers get together they

may be able to afford a vehicle to spray the pesticide.

The comparative risks of natural and synthetic pesticides can be hard to balance.

Applying synthetic pesticides gives good pest control — but at a high price in terms

both of cost and of toxicity to the farmer and the environment. Natural pesticides are

less toxic to the environment and may be cheaper — but they are still hazardous to

those who use them and less effective as pesticides, so make a more limited

contribution to solving the worlds food shortage. It is a dilemma which has not yet been

solved.

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It isn’t possible to eliminate risk form life completely – everything you do carries some element of risk,

however small. However in a chemistry lab you are more likely exposed to hazardous materials than

in other places. As a result, people in labs take precautions, use safety features and behave with care

to reduce the risks. Because of this laboratories are generally very safe environments. In contrast,

people jump into cars and drive every day with little or no thought for the hazardous nature of what

they are doing – and often make little effort to reduce the risks. There are a number of hazards

particularly associated with organic chemicals. Many of these chemicals are poisonous and/or

carcinogenic (they cause cancer). Many are flammable, As a result, when working with organic

chemicals it is very important to carry out a full risk assessment. The purpose of this is to identify the

risks of using hazardous chemicals and reduce them as far as possible.

To carry out a risk assessment you follow the steps below.

1 Identify any hazards of the chemicals you want to use and the procedure you plan to carry

out.

2 Quantify the risk associated with the way you want to use the hazardous substance, eg how

much of the hazardous substance do you need to use, will it be used by trained personnel, are

- fume cupboards available?

3 Identify who is at risk.

4 Identify any control measures which you can put in place to reduce the risk.

5 Quantify the risk that remains and decide whether it is now at an acceptable level. If so record

your risk assessment and carry out your procedure

Providing food to feed the population of the world is not an easy task, made more difficult by the fact

insects and fungi destroy up to 30% of the food that is grown. To prevent insect pests from destroying

crops as they grow, farmers around the world use pesticides — chemicals that kill the pests.

There are two main classes of pesticide used. Natural pesticides are derived from plants. Many

plants have evolved chemicals in their leaves and flowers that are toxic to insects and other pests.

People have extracted some of these chemicals and used them as pesticides. The best- known

natural pesticide is pyrethrin, an organic compound originally extracted from pyrethrum flowers.

Chemists have made a number of closely related chemicals that we even more effective. Natural

pesticides like pyrethrum e usually broken down relatively quickly in the environment both by sunlight

and within the bodies of many animals. Pyrethrum is also effective against malaria mosquitoes, which

cause disease and death to millions of people every year. However, there may be health risks to the

people who use it — in the US it has been classified as a likely human carcinogen by the

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Environmental Protection Agency. Because low levels of pyrethrin are broken down in the human

body, the best way to reduce risk from the pesticide is to spray the lowest effective amount.

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Hazard and Risk