I'm not involved in occupational hygiene, I'm a statistician, I joined this forum just to ask this question of some people who I hope might be experts in the field.

I've been doing some modelling of environmental contaminants and comparing the mathematical models to the epidemiological data on whatever illnesses they cause. I came to do some work on asbestos but found a huge discrepancy that I couldn't explain with the figures.

Basically, without going too heavily into the maths, any Acm (even ones with quite a low proportion of asbestos are made up of potentially hundreds of billions of fibres in every cubic centimetre. So any fully destructive activity (say drilling a hole) creates potentially hundreds of billions of fibres. It must do, because, being a hole, the stuff isn't there any more.

Now the poor worker drilling the hole might be exposed to something up to 100f/ml,but the air that effects can only be so big (say 2x3x3) which is millions of millilitres, multiply that by the 100f/ml and you get hundreds of millions, a thousand times out at least! So, where have all the fibres gone?

1. They fell straight to the floor in big lumps. That explains the air concentration, but then all those fibres are still on the floor. Hundreds of billions of fibres is enough to contaminate a whole house at millions of fibres per square centimetre. Even using conservative values for how many get put back into the air, every 1960s flat and office block would have been a deaths trap for years, way more than the generally very low air concentrations found in public buildings.

2. They're lost to air changes. Problem is we've just established that they didn't end up in the air, they weren't in the original disturbance otherwise the air concentration would have been higher, and they're not getting entrained into the air later otherwise the air in public buildings would be much worse than it is.

3. They got cleaned up. But conventional cleaning methods don't work with asbestos, they just spread it around and make matters worse.

So, as per the question title, where did all the fibres go? Every drill hole, every cut, scrape and knock throughout the 60s and 70s must have made trillions of potential fibres and yet public buildings even with damaged ACMs routinely show very low air concentrations.

I'd be really grateful if anyone with expertise in this field could answer this puzzle for me.

Thanks for taking the time to reply. I'm not sure I quite understand what you're suggesting, if you could spare a little more time to explain, I'd be very grateful.

What I'm having trouble understanding is the discrepency in numbers between the fibres released from a something small like a single drill hole (which could potentially number in the billions since the fibres are so small) and the air concentrations actually found in buildings even with damaged ACMs (regularly below 0.0003f/ml). I see what you're saying about investigating the asbestos history of a building and that might be a good route to go down for more accurate figures, but surely every building must have had at least a few holes drilled in it's ACM at some point in time, and yet no building to date has yet been found to have more than a few thousandths of a fibre per millitre of air, so I don't immediately see how the actual building history might help explain this discrepancy.

Secondly, I should have made it clearer in the actual post for which I apologise, but being a statistician I'm interested in the modelling of the processes, not any actual real life work. The time period I'm interested in is the 1960s and 70s, there would have been no H-class vaccum cleaners and no air monitoring to ensure safe re-occupation and yet the epidemiological data for non-occupational exposures during the 60s and 70s does not match with even a moderate exposure level. Out of all the people who went to school in CLASP buildings, worked in asbestos-ceilinged offices and lived in high-rise flats (which must be close to the entire population, especially when considering schools) less than 200 a year went on to develop asbestos related diseases fourty years later. Now at this point I should re-iterate I'm interested only in the statistics, 200 is still 200 too many and I applaud the work that's being done to reduce this figure, but 200 out of 700,000 deaths is a risk of just under 3:10,000, which, according to Hodgson and Darnton's estimates would equate to a lifetime cumulative exposure of about 0.1f/ml (say twenty years at 0.005f/ml).

Without further explanation (which is what I'm looking for here), that would mean that during the period when people took absolutely no precautions whatsoever with asbestos, the average air concentration arising from this lack of any precaution (after the dust had settled) must have been no more than about 0.005f/ml, and this ties in well with the few air tests done close to that time (1982 is the earliest I've been able to find). Even with conserative estimates of re-entrainment to the air from settled dust, ten billion fibres is enough to contaminate a whole building and a hundred times this figure and that's just one drill hole, hence my confusion about where all the fibres went.

Any further help understanding this would be greatly appreciated.

Hmm. Interesting question. Surely they all ended up in the ground, first as 'dust' then part of the soil, either via landfill as sweepings, building debris after demolition, or settling from the air to be integrated with the land by the actions of water, weather etc. As you say, there will have been an awful lot of fibres floating about over the years, so I too wonder as to the lack of evidence of widespread effects in those decades. Maybe people died of other things (smoking etc) before asbestosis had a chance to show, or medics didn't look for, or notice it with everything else in the air - smog, coal & cotton dust etc.

Once part of the soil, fibres are unlikely to refloat into the air where they would pose a risk, and so become part of the landscape; back to nature - from whence they came.

Thanks to all involved. I'm slightly concerned that there's no easy answer to this. Considering that in the event of a small asbestos incident or damaged ACM discovered, one might well be advised to spend hundreds, if not thousands of pounds to get it professionally sorted, I'd expect there to be some fairly clear demonstration of the risk that just leaving it would entail, but perhaps it's still out there somewhere.

To those suggesting it will simply end up on a ground, a little maths (only a little). Ten billion fibres  spread out over a floor space of 5x5m would give 40,000 fibres/cm2. Conversion factors (K factors), can be used to estimate re-entrainment from surface to the air. Most experiments concur on K factors for asbestos of 1e-5 (1 with five zeros in front of it). For our 40,000 fibres/cm2 this would give 0.4f/ml. Without any clearance (the dust is on the foor now so air clearance is negligable and apparently ordinary vacuum cleaners don't work on asbestos) that's 8f/ml.yr cumulative for someone living there for twenty yrs. This is higher than most pre-regulation industrial settings, just from one hole!

To those who suggest the fibres remain in clumps, that is exactly the explanation I presume must be true as I can't think of anything else, but that would have to come along with the rather controvertial fact that normal houshold cleaning methods (vacuum, duster, mop etc) are actually quite effective at removing asbestos as the small fibres which could theoretically make it through these processes don't really form a great proportion of the dust which is mostly made up of large clumps that the vacuum cleaner would both pick up and (largely) retain in it's bag. If we postulate that normal cleaning methods break up these clumps to dangerous small fibres, we're back to the problem again of where they all are in buildings which consistently show very low air fibre concentrations.

Postulating that normal cleaning gets rid of most of the fibres within a fairly short space of time (or even that some other natural methods such as air changes do) is, however, contrary to all advice on dealing with damaged or friable asbestos in the home (broadly, that it presents a risk high enough to be worth spending hundreds of pounds eliminating), hence my reluctance to accept this conclusion and my continued search for some other explanation.

Originally Posted by: ryangavin777

Would you not need to know how much asbestos is contained in each material? Presumably different materials contain different amounts of asbestos? Do you not also need to consider the compound in which the asbestos is mixed with?

Yes, for anyone interested, in my calculations I'm using a solid asbestos density of 2000kg/m3, density of AIB as 700kg/m3 and consisting of 30% asbestos, this gives a conversion factor of about 0.11. With an average fibre size of 0.16 cubic microns we get about 6e11 fibres per cc of AIB. I've used figures from air tests in asbestos mines which show about 2% of all fibres in the "dangerous" range of sizes, hence about 1e10 (ten billion) "dangerous sized" fibres per cc.

I take your point about the binding material, but as per my post above that would only lead to the the conclusion that normal household cleaning activities are basically safe for small-scale exposures (by safe, I mean as safe as crossing the road, or driving to work), which is definately not the message we're getting from the experts so I remain confused.

Interesting discussion

I suspect that after the initial disturbance, the fibres would settle and the airborne resuspension of fine particles/fibres from a surface is low thereafter, as small particles adhere to surfaces quite well.

Even if considering resuspension and associated resuspension factor (K). Typical K factors for walking on surfaces are around 10-5 (i.e. about 1 in 100,000 of the fibres would become airborne).

Regards

Originally Posted by: Hilti

Even if considering resuspension and associated resuspension factor (K). Typical K factors for walking on surfaces are around 10-5 (i.e. about 1 in 100,000 of the fibres would become airborne).

Yes, those are the k factors I'm using, but extrapolating back from them to 0.0003f/ml gives 30 fibres/cm2 multiplied by a 10x10 office that gives 30,000,000 fibres in settled dust. Seeing as a single hole could contain 30,000,000,000 fibres, where have all the rest gone?

Originally Posted by: hilary

But then, perhaps I completely missed the point ....

Yes, a bit, to be fair it is now abit buried in the post. I'm talking about the fibres that would have been created either in the 60s and 70s when no precautions were taken, or nowadays in situations where no-one involved is aware that the material is an ACM. In neither case do we find any evidence of greatly elevated air concentrations afetr a period of time, yet there doesn't seem to be any reconised means of fibre removal. So where did they go?

Originally Posted by: Hilti

In real world tests where a 7 mm dia 1/4 inch drill bit was used to make a hole in AIB. This resulted in 0.35 f ml-1 to 0.9 f ml-1. This gave an average of 1,800 airborne e fibres during the activity. 36 fibres being in the dangerous range.  

OK so presuming it takes only a matter of seconds to drill a hole, we can eliminate the effect of any of those fibres falling to the floor under gravity as that would take hours so a 7mm hole in 25mm board has removed almost exactly 1cc of board (0.35^2*3.14*2.5), about 30% of which is asbestos depending on the type of board. 1800 fibres were airborne which at 0.16cubic microns per fibres is about 300 cubic microns, so out of 1e11 cubic microns of material (1cc, adjusted because the material is not 100% dense) 300 of which are airbourne sized fibres, the rest must be either single fibres or clumps mixed in with the binding material too big to be airborne (otherwise they would appear in your air tests).

The fate of these fibres is what I'm interested in. Over time, they cannot create more than about 0.0003f/ml concentrations otherwise the air tests in public buildings would generally be much higher and many more non-occupational deaths would have occurred from the 60s and 70s.

Using k factors of 1e-5 this would allow a maximum of 3e6 fibres in a 10x10 office/classroom (any more than that and the air concentration would be higher than has ever been recorded).

But 1e11 cubic microns of asbestos can potentially yield 6e11 fibres. To get from this potential figure down to 3e6 by normal cleaning methods over a relatively short timescale would mean that normal cleaning methods would have to be 99.9995% effective at removing asbestos fibres (or potential fibres all clumped together), That would mean even if we were to say that this was the result of 1,000 such operations over time (about one per day for three years), each one would still be 99.5% effective, which is a conclusion that flies in the face of all the advice from experts so can't be right, but I'm still not seeing anything else to explain where all the (potential) fibres went.

And just to keep perspective, we're still just talking about a single drill hole, how many holes, cuts breakages, sandings and general damage must a standard school/office have suffered during the 60s and 70s when no-one paid any attention to the dangers of asbestos? Yet they've still all ended up with only 0.0003f/ml in the air (about 30f/cm2 settled).

Ian, no, I'm very much much unsure that my assumptions are correct (far more certain of my calculations, as this is my job) that's mainly why I'm posting here to see if anyone can correct me on them.

Dan,

You may be oversimplifying things. The ,methodology for analysys and determining fibre count is given in MDHS 39/4.

When you've digested that, can you come back and explain it? It's baffled me for Years!

Originally Posted by: Ron Hunter

You may be oversimplifying things.

I've read the methodology, I've digested quite a lot of data in my research on this topic, I'm not quite sure what you might mean by linking the fibre counting methods to the possibility that I might have oversimplified things. If you are suggesting that most of the billions of fibres that could have been released are too small or too large to be counted, then yes, I had thought of that, but the numbers would be massive.

Take for example an acitvity which generates a measured air concentration of 50f/ml (countable fibres). In order to do that over a dispersal area of say 2x3x3, it would have to generate 3e7 fibres per minute (this is to fill the space and to keep it full despite the fact that fibres are settling to the ground at a rate of 0.3mm/sec), so fifteen nimutes of such activity would generate about 4e8 countable fibres. Cutting AIB with a power saw might be just such an activity, but cutting AIB with a power saw (presuming 8ft boards and a 1mm kerf) would generate 3e13 potential fibres (i.e of all sizes) per cut. 15 minutes of such cutting would produce at least ten times as many (I don't know exactly how many boards one could cut in fifteen minutes). This would mean that out of 3e14 potential fibres only 4e8 are of countable size, that's about 0.00001%. I've no reason not to belive that to be the case as I've no expertise on how asbestos breaks up (that's why I'm here), but if it were true that only about 0.00001% of any asbestos fibres released by damage are of countable size, then that would have serious implication for asbestos management as per my post above. Either these other sizes are not dangerous and so minor asbestos incidents really can't release enough dangerous fibres to be of any concern, or the premise on which non-occupational asbestos related mesothelioma is based has to be questioned.

Originally Posted by: Bigmac1

There is between 10 and 200 fibres in the 1m3 of air we breath depending where you live. Not enough to cause serious health problems for the public, hence the lack of non-work related cases. Its at the workface we still have the problem. Lets increase awareness of all lung disease, get behind the BOHS Breathe Freely campaign or IOSH' No Time To Lose campaign.Spread the word people too many people are dieing when they shouldnt be.

I agree with your sentiment entirely, but have you read the HSE report about why people are not heeding the message about the risks from asbestos, conflicting messages and a culture of not believing in the risks come out pretty much at the top and a "don't look too closely at the figures" kind of attitude at HSE is not going to help. If we really want to save the lives of the people at the coal face who really are at risk, there needs to be openess and honesty otherwise the message will just not be believed. Admittedly I'm a mahtemitician so I'm comfortable with the figures, but if I can calculate a huge dicrepancy, then I'm sure many builders, electricians etc can see it too, even if only intuitively. If one message coming out of the safety industry is clearly contradicted then people are much more likely to throw the whole lot out and just carry on as they were. That's one of the reasons I think it's so important to get it right, so that the people who need it can really believe in it.

Dan, I am not a number cruncher so I readily admit I do not fully appreciate the sums or hypotheses you are promulgating with regards to exposure from asbestos contaminated materials. However, I do understand the concept of risk, subjective though it is, I suspect others like yourself are not so familiar with this concept.

The HSE have provided much guidance on how asbestos should be managed to try and reduce the risks from exposure. There are many different types of ACMs, some are by definition low risk and some are high. The condition for each and every type of ACM needs to be assessed as well as the location and other factors. So, there are many variables which need to be taken into account when managing ACMs. Notwithstanding the risk element, there is also the matter of cost, which is no small matter.

Your premise that h&s practitioners are not properly managing the risk from ACMs I would challenge. We may not be able to quantify those risks in a scientific formula, we have many other non-asbestos related risks to manage with limited resources and budgets. There is no such thing as a risk free society - it is about managing those risks to as low as reasonably practicable.   

Dan

Not sure what geographical area you are referring to as regards road traffic fatalities.

But latest stats to end March - 1780 in Great Britain + 68 in Northern Ireland equates to 35.5 deaths on the roads per week, so does not translate to 140 work related road traffic fatalities per week.

I've never seen an authoritative estimate of how many of these deaths are work-related.

In the past HSE has estimated 10%, RoSPA 25%, Devon and Cornwall Police 40% in their patch.

Partly the lack of clarity has been HSE's consistent position that we should be one of the few developed nations that do not require the reporting of work-related transport accidents (with very limited exceptions).

My guess is that perhaps half of those killed or seriously injured on the roads are work-related. But this would only mean 15-20 work-related road deaths per week. (It would seriously impact on the U.K.'s position on the EU league when looking at accident rates.)

As far back as the 1st Century, Pliny advised villa owners in Cyprus not to take on former quarry slaves as "they tend to die young". Cyprus did not stop quarrying asbestos until 1985.

Originally Posted by: peter gotch

My guess is that perhaps half of those killed or seriously injured on the roads are work-related. But this would only mean 15-20 work-related road deaths per week.

Yes, should have read 15, a stray zero, unforgivable for a matehmatician, I hope my clients aren't reading this! Point still stands though even at 15.

Originally Posted by: Xavier123

I would have thought that the better people to ask would be ATAC and ARCA - the relevant testing and removal associations.

Yes, you'd have thought so wouldn't you. I've written to ARCA, HSE, HSL, PHE, the occupational health units at several universities, and the IOM. No response at all from any of them except HSL, who wrote back simply confirming what I had speculated, namely that the average fibre concentration in ACM containing building was indeed 0.0003f/ml and that no, no-one had ever tested the fibre release from small scale domestic exposures, including settled dust nor the effect of household cleaning activities. Did they have any intention of doing anything about this gap in the evidence base? No.

Just another thought on the same topic for anyone interested. Air tests showing that buildings have below 0.0003f/ml regardless of the condition of the ACM are not being taken as indicators that the building is safe, the advice on survey and management is still to remove or encapsulate damaged ACMs regardless of of the cost (i.e. not cost benefit analysis is carried out), or scale (there is no maximum size of material that can reasonably be left untreated. This implies air tests are not being considered as indicators of the safety of the building. Where does that leave the reassurance test which only proves the air is below 0.01f/ml, if proving that the air is below 0.0003f/ml isn't enough?

Basically, how is it being justified to advise building owners to remove damaged ACMs when all the evidence we have would seem to indicate that leaving them in place causes no more than 0.0003f/ml whereas the best that a removal operation can promise is less than 0.01f/ml. Surely for small-scale asbestos these figures present an argument for leaving it in place even if it is damaged and even if it can't be encapsulated? What evidence does anyone have that removing small pieces of damaged asbestos that can't be encapsulated releases fewer airborne fibres than normal occupation activities with it still in place? If no tests have been carried out on small scale domestic interactions with asbestos, and no air tests in buildings with damaged ACMs have shown any elevated levels then there's absolutely no evidence that the removal of such items is actually safer for the public than leaving them where they are.

Originally Posted by: Xavier123

I have two additional thoughts/theories for the mix.

1. Asbestosis and related illnesses are currently still on the rise. Political will to make any lessening of control measures would be virtually unthinkable.  Any scientist knows that policy does not necessarily follow the science.

2. Exposure to asbestos fibres has no 'safe' level.  I recall there is a dose response curve but there is no WEL. Yes, fibres levels are demonstrated as being low despite historical damage to ACMs etc. but they ARE still found.  You've either got to declare a WEL (which they haven't got the empirical data to do as far as I know other than through circumstance) or prescribe more demanding controls. 

1. That's true, but asbestos related diseases at the scale of exposure we're talking about are all cancers and the rate of all cancers is on the rise. In fact the rate of asbestos related cancers is rising slower than the rate of many other cancers so in order to ascertain what this means we must control for whatever factors are causing all cancers to rise (mostly an ageing population). If we do this, we find (depending on the statistical model we use) that rates of asbestos related cancers are falling relatively. Another way of doing it is to look at rates in younger (but still adult) age categories, which again are falling on average. So the continued rise is not so much related to absestos exposure post-regulation, but simply the fact that people are living longer.

Following on from what you said about political will, I came across this poster during my research from BOHS which shows a shoking level of statistical ineptitude. It states "Fact: Asbestos kills around 5000 workers each year", "Fact:Asbestos is still a problem today - it can be present in buildings [constructed] up to the year 2000" and "There is a need for qualified professionals to minimise the risk". Whilst I don't doubt the final conclusion, it just plays directly into the hands of people wishing to play down the threat if they're going to use such blatantly false scaremongering to get their message across. To link the first two facts is appalingly bad science, they are totally unrelated, as we have established in this discussion, there is at present no evidence to prove that any of the 5000 deaths are related to there being asbestos in buildings at the levels currently experienced. If organisations like BOHS want to get people on board with being more aware of the risks from asbestos to people nowadays then they need to be campaining for some research to be done into that risk, which we currently seem to know nothing about, not the one that was present 50 years ago which is completely different from that which we have now. Personally I can't see anyone looking at that poster and not just thinking, well it doesn't apply to me, all those 5000 people were exposed years ago and things are different now.

2. This is true (if, and only if, you accept what's caled a linear no-threshold model for exposure, which even the authors of the model admit is a complete guess with no evidence to back it up), but this is also true of all genotoxic carcinogens, it's to do with the presumed mechanism of carcinogenisis. The issue I have is that someone's got to set a safe level, we can't all wear facemasks permanently because of the ambient level in the air. If the experts don't set a level based on the best data they have and weighed against the cost, then all that's going to happen is people are going to that calculation themselves with what limited and narrow data they happen to have. That's the sort of behaviour that puts people at risk.

Originally Posted by: Xavier123

Well, this has moved on from your original post....!

It's a reasonable drum you're beating but one that requires significant understanding of the question marks surrounding the genuine level of risk...which in turn sounds like it requires more directed research that might rail against the current direction of travel for the issue.  I think you've made some really good points about the evidence base here and got people thinking.

With regard your point about people taking more risk because they can't equate the facts you highlight with the legislative requirements, I'm less convinced.  The legislation has been around a substantial time now and is well understood, regardless of opinion concerning its necessity/depth.  In my experience, those who breach it do so through ignorance or recklessness, not out of reasoned calculation concerning relative risk levels. I would wager that the group you're concerned about there is pretty darn small.

Yes we do seem strayed off topic. All I can say is I hope you're right on both counts, but I can't in my experience justify that hope. There seems literally no interest in modelling the risk that we have nowadays from asbestos in buildings and part of my job is communicating risk to people and all that my years of doing so has taught me is that people do no understand risk at all and if you want them to take certain precautions you have to spell it out in detail. With asbestos, the HSE certainly seem to have identified what they think is a problem with people not treating asbestos with appropriate precautions and who hasn't heard that asbestos is dangerous? No-one I should think. That means that all the inappropriate activities the HSE have identified as a problem are being carried out because people are making their own guesses as to how dangerous. Even though I didn't have any involvemnt in the issue at the time, I still remeber the "one fibre can kill you" message (despite the fact that virtually everyone has a million fibres in every gram of lung tissue). People are not automatons and will not simply do as they're told without question. How many people still use machinery without the appropriate protection, or carry weights badly, even cross the road when the lights are still red? And we actually have current indisputable facts to prove these activities are harmful. To try and change people's approach based on some speculative guesswork from exposures in a completely different environmnet 50 years ago is ambitious to say least.