Air monitoring, where do you place the pumps?
The aim in undertaking air sampling is to get a realistic sample of the air that people are breathing. As this is a scientific process we need to ensure that we can as much as possible have confidence in our results.
However, prior to undertaking any air sampling there are two very basic questions that must be answered.
- Why are you sampling? We need to be honest about the reason for the sampling, this will assist in designing the sampling regime to ensure the results will match expectations.
- What are we sampling for? What we are sampling for will determine what type of sampling filter head is chosen.
Sampling set up
It is important that we sample the air that people are likely to breathe, and most of this air is generally located between 1m and 2m off the ground. This can be achieved via two methods:
- Attach it to one of the workers, this is generally achieved by attaching a filter head to the lapel of the worker.
- Static sampling, this can be achieved by placing the pump in a fixed location normally about 1m away from any wall or surface. When choosing the location ensure that the filter head is kept within the breathing zone. One way of achieving this is by attaching the filter head to a stake of known height.
It is also important that the filter is located at approximately a 450 angle. This serves two purposes:
- Our noses come out of our heads at about that angle, and we are trying to replicate that.
- If there is moisture around (e.g. rain) the filter will remain dry.
Sample locations
When choosing the location to sample consideration needs to be made as to why we are sampling. The main reasons are as outlined below (there are no doubt many more and these will also need to be considered):
Occupational monitoring
In this situation, the sampling locations are to be chosen to best represent the air that the people working or living in the area will be exposed to. At the same time however, we need to ensure that the sampling location will not interfere with the general daily activities.
We need to establish the following:
- How many sampling points are needed to get a good coverage on any area? For example one pump to cover a multi-level building will not give a representative result, whereas multiple samples in a small space will not provide any more information than a single sample.
- When should you sample? There might be different activities occurring at different times, for example if the dusty work is always done on a Monday, then taking a sample on a Wednesday will not provide information on the worst exposure levels.
- How long should the sample run for? Ideally you want the sample to run over a full working day, but for many reasons that is often impractical to manage so a shorter time is needed. Often a four-hour sample is deemed to be a reasonable compromise.
- What should the flow rate be? The required flow rate will differ depending on the sampling method chosen.
- How often do you need to run the sample? A single ‘snap shot’ sample does not often provide any certainty on the results, and therefore a programmed sampling run is required.
Process monitoring
With process monitoring we are trying to establish if the ‘process’ is affecting the quality of the air that people, this can be either:
- To confirm that the control systems are working
- To see if the process itself is affecting the air quality.
The aim of this Process monitoring is to ‘follow’ the movement of air through the process, and also to check for ‘leaks’ from the process.
In choosing sampling locations for checking systems the following places need to be monitored:
- Incoming air flow.
This is to check both the incoming air quality and to see if there is any leakage of contamination back against the flow, i.e. is the decontamination process working.
- Outgoing flow
This is to check that any air filtration, dust suppression etc measures are working and that there are no leaks.
Often the outgoing flow is via high volume air filtration units. It is important that the sampling location is placed near the discharge point to ensure that the sample collected relates to the discharged air. However, care needs to be taken that it is in the slip stream of the discharge flow. If the sample is exposed to the full flow then it is likely that the sample will not be representative.
If it is not possible to get into the slip stream of the flow, for example the air is discharged out a window at height, then a sampling ‘weep point’ can sometimes be placed into the discharge pipe. This will allow a small amount of discharged air to flow past the sample head.
If you are wanting to check the process itself, then you need to insert air monitoring stations within the work zone in a similar manner to occupational monitoring.
If anyone needs assistance in this area, then please do not hesitate to contact me.
While I am a Chartered Professional Engineer and an Authorised Signatory this blog is not written as professional advice, but rather a commentary on the topic that I believe may be of interest. Readers of these blogs will need to seek their own independent professional advice for any situation they are experiencing.