So what is an Airflow Indicator?

Good question. I’ll get to that in a minute.

Firstly, some background into how an employer can irreversibly damage an employee’s health by providing a dangerous working environment.

To quote the Health and Safety Executive (HSE) –

‘Thousands of British workers contract occupational asthma and other lung diseases each year. Many people die or are permanently disabled by these conditions and are unable to work. People develop these diseases because they breathe in too much dust, fume or other airborne contaminants at work, often because control measures do not work well enough. Most industries are affected, including woodworking, welding, paint-spraying, stonemasonry, engineering and foundry work.’ (HSG258 para 1)

Employers are bound, under the regulations of the ‘Control of Substances Hazardous to Health’ (COSHH) Act 2002, to put in place control measures to minimise employee exposure to inhalable and respirable substances, and hence attempt to reduce long term ill health.

What are these Control Measures?

There are various levels of control available to the employer to reduce the employee’s exposure to hazardous substances, including –

  • Eliminating, replacing or reducing the hazardous substance and
  • Modifying the process to reduce the frequency or duration of emission

If these measures cannot adequately reduce the level of airborne hazardous substances, then other measures, such as LEV need to be employed.

What is LEV?

Wherever hazardous airborne substances are present  a COSHH assessment needs to be carried out. If the result of this assessment is that potentially hazardous substances cannot be sufficiently reduced by the above controls, then one common way to reduce the risk of inhalation is to remove the substance from the air around the employee. The normal way to do this is to use Local Exhaust Ventilation, or LEV – sometimes known as Industrial Ventilation.

LEV is an engineering system designed to reduce employee exposure to airborne contaminants in the workplace by capturing the emission at source and transporting it to a safe emission point or to a filter system. The LEV works on the same principle as a vacuum cleaner. A fan is used to generate a negative pressure (partial vacuum) in a length of ducting, which draws the contaminant-laden air, via a hood, into the ducting and through the fan. Filtering may or may not be carried out on either the positive or negative side of the fan. The result is that air around the employee is replaced by surrounding air which has not been contaminated and is safe to breathe.

What Can Go Wrong?

A well-maintained LEV should carry on protecting personnel for many years. To ensure that the system is working as it should, a Thorough Examination and Test (TExT) needs to be carried out at least every 14 months, possibly more frequently if the substance requires it. The essential parameters of the system are tested and measured, and the whole system inspected for damage. This process needs to be undertaken by an experienced inspector in line with HSE guidelines and a thorough knowledge of the science behind LEV. Hopefully the system will then carry on working until the next inspection is due.


An LEV is a large and complex piece of equipment which relies on every part continuing to work perfectly as specified and designed. If any part of the LEV gets damaged or fails to perform as it should, it can affect the ability to carry away the contaminents and hence protect personnel.

Factors that can affect LEV performance

  • moving parts that may wear, such as fan bearings or filter shakers;
  • non-moving parts, such as hoods, ductwork and seals (which can suffer physical or chemical damage and wear);
  • parts that deteriorate with use, such as filters or flexible ducting
  • items that need regular attention, such as filters that need replacing, or removing
In addition to the above are such non-predictable incidents as collision damage (fork-lift truck etc)  and paper or cloth being sucked into the duct and causing a blockage.

So how can I ensure that my LEV is working correctly ?

One way to retain faith in your LEV is to check its operation more frequently, using an anemometer, manometer or a combination of both. However often you check, though, and how many man-hours you throw at it, something can still change when your back is turned.

If you want to know, at any time, how well your LEV is performing fit an airflow indicator.

So what is an Airflow Indicator?

An airflow indicator is a device that continually monitors the static pressure in a duct. Since duct pressure is linked to air velocity, this gives a true indication of how well the LEV is working. Airflow indicators have, in the past, been based on standard pressure gauges manufactured for the low negative pressures found in common LEVs. Modern airflow indicators are based around solid-state electronics and pressure sensors to provide a clear, easily-readable solution.

The Monitair Chevron is a hi-viz intuitively designed electronic airflow indicator. To learn more about it click here.