Something in the air
June 1st 2007

We assume, because we cannot see any thing in front of our noses, that the air we breathe is pure and harmless – and for the most part it is – but air has the ability to hold a variety of substances, often invisible to the eye, that can cause health problems if inhaled, explains John Horsey of Purex International

What substances could we be breathing in while working? Airborne contaminants can be: Dust – produced by industrial processes either project powdery products into the air or cause solids to be broken down into small particles Aerosol mists – liquid droplets formed where a fluid is broken down into a finely divided form Vapours – evaporated from the surfaces of liquids, these behave in a similar way to gases but can condense into droplets Gases – along with vapours, these are responsible for the odours which characterise many processes

Harmful Effects ? The effects of breathing in particles vary from minor respiratory irritation, coughing, breathlessness, bronchitis and rhinitis to the more serious occupational asthma, and even to the extent of heart disease and lung cancer.

Who protects the Workers ? The Health and Safety at Work Act 1974 and the COSHH Regulations provide overall protection for employees against general hazards in the workplace including the inhalation of harmful substances. The HSE publication EH40 (ISBN 0-7176-1315-1) lists allowable levels of airborne contaminants.

Keeping Workers safe The COSHH Regulations list the priorities for actions to be taken to control airborne emissions: 1. Change the process to prevent the dust, fumes or gases from being generated in the first place 2. Provide fume extraction to remove airborne contaminants This is the most practical and effective method of controlling airborne emissions. 3. Provide personal protection equipment

Fume and Dust Extraction Properly designed dust and fume extraction systems prevent the escape of fumes and dust into the workplace, and also the contamination of equipment associated with the process and the product . This involves enclosing the process as much as possible and engineering enclosure design to make sure the extract air flow is used most effectively.

Filtration Equipment Equipment available for filtering the contaminated extract air for dusts: Wet systems include self induced spray wet collectors, venturi scrubbers, and spray chambers. Each uses the principle that a scrubbing fluid is broken into a finely divided form and the resulting tiny droplets are brought into contact with dust particles. These units are only effective on relatively large particles.

Dry Systems can be self cleaning type where filters are shaken or cleaned with pulses of compressed air, or electrostatic precipitators.

Disposable Filters consist of multipocket prefilters with HEPA filters to remove > 99.9% of particles from the airstream so that the extract air can be returned to the workplace.

Choosing a Filtration System: If the dust burden is low and there are very small particles or droplets present which are 'sticky' in nature then disposable filters will be preferable. If the particulates are large and sticky and the dust loading high then 'wet ' collectors will be necessary. Heavy burdens of dry dust are better removed by self cleaning filters but many dry dusts can be explosive so care in selection and design is necessary.

For Gases and Vapours: Gas Scrubbing .This involves passing the gas or vapour through a chamber filled with 'packings' which are irrigated with a scrubbing liquor. These create a large number of sites where the liquid and gas can interact.

Chemisorption. The gases are passed through a bed of a substance that will chemically react with the gas e.g. oxidising agents.

Adsorption .The gases are passed through a bed of a porous substance, with a high surface area e.g. activated carbon. Molecules of the contaminant gases are attracted to carbon molecules on the large surface area and are held in the porous structure.

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