The price, but also the quality of an asbestos survey depends to a large extend on the number of samples taken. Various organisations have tempted to give recommendations on the number of samples to take. If we compare these figures, we realise however that these recommendations diverge considerably. It appears that the experts are far from agreeing on this point. But how can we get to a better approach?

Note: The author of this article is not native English speaking. So please be kind with your critique on form and grammar.

Last year I was involved in a project where we had to survey buildings with a total surface of more than 200’000 m² (2,000,000 ft²) . The building owner initially estimated that several hundred samples needed to be taken. In the end, there were however more than 1000 samples that needed to be analysed. Justifiably the building owner asked whether so many samples are really necessary. We then compared different standards and recommendations. The results are summarized in the following table.
 

		VABS	STIPI	EPA	AFNOR	HSG264	VDI
Flooring	1 room, 10 m2	1	3	3	1	2	-
	10 rooms with 10 m2, same material	8	3	5	1	-	-
Tile glue	1 room, 10 m2	1	1	2	1	2	2
	100 rooms with 10 m2 , same tiles	8	3	-	-	-	6
Tube insulation	20 m	1	-	-	1	7	-
	200 m (same type/ diameter)	-	-	-	1	67	-
Ceiling acoustic insulation	1 room of 25m2 with 100 tiles	25	1	3	1	-	-
	10 rooms of 25m2 with 100 tiles each	30	7	-	1	-	-

Switzerland: ASCA

The Swiss Association of Asbestos Consultants ASCA uses a statistical model: The calculations are based on the “homogeneous unit”. In the case of ceiling tiles for example, each tile is considered such a unit. The goal is to take enough samples so that you get a 75% probability that you find asbestos even if only 5% of the tiles contain it.

This method is interesting as it is based on calculations and not just on “good feeling”. But what is a “homogeneous unit”? And what about this hypothesis that 5% of the tiles contain asbestos? This approach still leaves a lot of autonomy to the person conducting the survey and generally leads to a very high number of samples.

Geneva: STIPI

The recommendations for building surveys of the ASCA are themselves based on recommendations from the authorities of Geneva (formerly STIPI, today called SABRA). This document is not in use anymore. It is mentioned here because the figures given there are a quite good representation of the common practice in the German part of Switzerland (in the French part, it's much more the ASCA recommendations that are followed).

 USA: EPA 

In the USA, the Environmental Protection Agency EPA published recommendations regarding asbestos surveys in 1985 (EPA 585 / 5-85 030 a, also called “pink book”). This document does mainly concern friable asbestos and requires at least 9 samples, except in small rooms:

• 3 samples for rooms of less than 1000 ft2 (92m2)
• 5 samples for rooms between 1000 ft2 (92m2) and 5000 ft2 (464m2)
• 7 samples for surfaces that are bigger than 5000 ft2 (464m2)

For samples of non-friable materials the document only speaks of “samples” in plural. It is thus assumed that one has to take at least 2 samples.

 Fr ance: Afnor 

In France, the AFNOR standard NF X 46-020 describes how to conduct an asbestos survey. This standard says that the person conducting the survey has to decide her or himself how many samples are required, but that normally it should follow the recommendations given in this standard itself.

For the floor tiles as well as for acoustic ceiling tiles, this standard says that 1 sample has to be taken for every type of product. This can be interpreted as if even for 10 rooms with acoustic ceiling tiles only 1 sample needs to be taken.

England: HSG 264

The English standard HSG 264 gives indications about the number of samples for a small number of materials. For homogeneous materials, the standard requires 1 sample, but for example for tube insulations, the standard explicitly requires one sample every 3 to 6 meters.

Germany : VDI

There is no German standard about asbestos building surveys. A working group of the Association of German Engineers VDI has published a paper in summer 2016 that contains data for certain materials. A new standard on this topic should be published in the future.

Interpretation

The diverging data shows mostly one thing: We are far from agreeing how many samples we should take.

Now we could do two things: We could either go towards more details, but with the risk that we create something that is so complex that it becomes unusable for the practitioner. Or we can say that in the end every specialist has to decide him or herself how many samples to take.

As a person that trains people in doing building surveys, it is for me not an option to simply say: “take as many samples as you think is necessary”. We need a more detailed approach.

But how to get there?

Criteria to determine the number of samples

Risk based approach: The first point to determine is: What is the reliability that we want to achieve or: How many accidental asbestos exposures (of workers or of people living in a building) do we consider as acceptable? Do we aim for a zero risk? Then we’d have to take a very high number of samples. Or do we say that a reduction by a certain factor is enough? If I say for example that a 99.9% reduction compared to exposure levels of the seventies is enough, the number of samples is much lower.

Homogeneity: The various standards agree on this: The number of samples to take depends heavily on the homogeneity of a material: The more heterogeneous it is, the more samples are necessary.

Reliability of the laboratories: The requirement of the EPA is presumably based on the hypothesis that there are errors in the laboratory. As those working in laboratories are just humans, it would indeed be wrong to assume a 100% reliability. But how high is this reliability? Is it still justified that we always have to take at least 2 samples or did laboratories get better over the last three decades which would allow fewer samples?

Asbestos content: This is linked to the reliability of the laboratory: For materials with a high concentration of asbestos, the number of errors is probably much lower than for materials that contain only a few tenths of a percent of asbestos like window putty.  

Friable/non-friable: A tar-based material that contains asbestos is much less dangerous than for example gypsum based materials. It is thus less of a problem if I such a material is accidentally declared as asbestos free.

Others: The goal of this article is rather to launch a discussion on this topic than to give the ultimate solution. Thus any complement is welcome.

Statistical approach

Through all these considerations it becomes clear: There is no simple approach for all materials. If we want to give recommendations for the sampling strategy, we have to differentiate (while still trying to stay simple enough that things are useful for the practitioner). We have to collect all kinds of data and knowledge from experts, for example about the risk homogeneity or the asbestos content.

Such an approach based on data and experience can be combined with a statistical approach: What is the risk level that we accept? Do we aim at zero risk? And how do we reach this goal?

Here is an example: The ASCA requires a reliability of 70% percent even if only 5% of the homogeneous units contain asbestos. If we consider one single flat, this value may seem rather low. But if we consider a whole lot of similar buildings, the actual exposure of the workers can already be massively reduced even with a small number of samples:

Example: 100 ceiling acoustic tiles (visually similar) in 100 different buildings. (The estimations given below are based on information received informally from several laboratories. The magnitudes are probably not too far off, but questioning these figures is absolutely right).
	Number of samples	Reliability
In 1% of the buildings all 100 ceiling tiles contain asbestos.	5 samples	If all tiles DO contain asbestos, theoretically one single sample would give a 100% reliability.
In 5% of the buildings, some ceiling tiles contain asbestos and others don’t. Let’s make the conservative hypothesis that only 10% of the tiles contain asbestos and 90% don’t.	5 samples	41% (calculations based on ASCA framework)
In 94% of the buildings, none of the ceiling tiles contains asbestos.	5 samples	100%
Average reliability		97%

Conclusion

The goal of this article was mainly to show that we are far from agreeing how many samples are actually necessary and that – if we want to become more concrete here, we’d have to consider different factors and different approaches, including things like the homogeneity of a sample, the reliability of laboratories etc. An answer on how all this factors can be integrated can not be given here. I hope that this article can contribute to a brother debate on this topic.

One final remark: In the case mentioned in the introduction, it was decided to strictly follow the ASCA approach, which required way more than 1000 samples. The interesting point in this: It allowed to declare certain elements asbestos free that in a first step (with fewer samples) had been considered contaminated. The high number of samples may thus actually reduce the cost of asbestos removal. Higher costs for analysis may thus overall not necessarily be more expensive.