A reassurance air test pulls four hundred and eighty litres of air through a filter and then a person counts the fibres on it by eye down a microscope. The pass figure is one hundredth of a fibre per millilitre. That room gives me eight thousandths, every time, and it has been stripped to brick.
I am an analyst. I do four figure clearance certificates on asbestos removals, mostly in schools in the holidays.
What the job is, because the method is the point.
After a removal you do a four stage clearance. You check the paperwork, you go in and inspect the enclosure visually on your hands and knees with a torch, you do an air test, and then you check the site is safe to reoccupy.
The air test is a pump pulling air through a filter for a measured time at a measured rate, usually eight litres a minute for an hour, so four hundred and eighty litres. Then the filter is cleared and mounted and you sit at a phase contrast microscope and count fibres in a set number of graticule fields.
There are rules about what you are allowed to count. A fibre is longer than five microns, thinner than three, and at least three times as long as it is wide. Anything else is not a fibre for this purpose, whatever it is.
The pass limit for a reassurance test is nought point zero one fibres per millilitre. Clean air in an ordinary building gives you below the limit of quantification, which is a fancy way of saying you count nothing and report less than.
Room 7 in the science block gives me nought point zero zero eight.
Under the limit. A pass. I have issued the certificate every time.
Nought point zero zero eight means I counted fibres. Not one. On a four hundred and eighty litre sample at that concentration it is about five or six fibres across the fields I am required to look at, and five or six is a long way from a dust artefact.
There has never been asbestos in that room.
That is the sentence that has cost me four years of thinking about this.
The removal in 2019 was in the room next door and the corridor ceiling. Room 7 was the clean side of the enclosure and my test in there was a background sample, which is what you take to prove the enclosure held.
The survey for the whole block is in the file. Room 7 has no asbestos containing materials in it and never did: the ceiling is plasterboard, the walls are brick and plaster, the floor is vinyl from 2011 with a certificate of analysis on the old one saying it was clean.
In 2021 that room was stripped for a refurbishment. Ceiling out, plaster off two walls, floor up. I did the air tests for that job too. It gave me nought point zero zero eight with the room down to brick and joist.
What I have done, and the first three are what anybody in my trade would do.
A field blank and a lab blank with every single test, which is standard, and both clean.
A different pump and a different filter batch. Four times. Same.
A different analyst. This is the one that mattered to me personally, because a fibre count is a human judgement and I have been doing it for nineteen years and I know exactly how easy it would be for me to be the fault.
My colleague counted a room 7 filter, blind, among eleven others from three sites, and reported nought point zero zero seven on it and a less than on the others, and she has no idea to this day which room that was.
What the fibres are.
I sent three filters for analysis by electron microscopy, which the company paid for once and I paid for twice, because transmission electron microscopy will tell you what a fibre is made of rather than just that it is there.
The report on all three says the countable fibres are amosite.
Amosite is brown asbestos. It was not used in that building. The survey for the site lists chrysotile in the corridor ceiling and the boiler house, which is what you would expect for the date, and there is no amosite anywhere in the survey, and the removal next door in 2019 was chrysotile with the consignment notes to prove it.
What I do about it.
I report the figure and I issue the certificate, because nought point zero zero eight is a pass and my job is to apply the standard, not to have a feeling about a room.
I have written it up as an anomaly twice in the comments field, which nobody reads, and I have told my technical manager, who said to keep the blanks tight and to watch my clearing technique.
The school has had twelve hundred children through that room since 2019 and I want to be plain that, on the numbers, that is a room with cleaner air than most of the streets they walk in on.
The only thing I have changed.
I used to put the pump on the bench in the middle and leave the room while it ran, because you do not want your own movement stirring the place up. An hour in the van with a flask.
Now I set it at the far end by the window, because of something I noticed when I plotted the filters.
The fibre count on a filter is spread across the fields you look at, and the spread is random, and you can see that it is random because you note where on the filter you found each one.
On nine of the eleven room 7 filters, the fibres I counted are in the fields on the side of the filter that was facing the door.
A filter does not know which way it is facing. It is a disc of cellulose in a plastic holder and the air comes at it from everywhere.
So I put the pump by the window, and I record the orientation on the bag now, and the last two filters did the same thing, and the door is where it is.