This lab smells….

One of the key ideas behind the science of Why Scotland, Why East Kilbride is the use of chemicals to allow the audience to get some of that ‘authenic lab olafactory experience’! There have been lots of obstales and this is how we have got round them.

Safety and Odour threshold levels
For this to work it has to smell, but also be safe. There’s a big list of odour threshiold levels at this link (it’s entitled “3M Occupational Health and Environmental Safety Division, 2010 Respirator Selection Guide” if the link dies). It also contains occupational exposure levels, but UK WEL’s can be easily found from Material Safety Data Sheets by google. You want the maximise the ratio of the odour threshold to the safety threshold. The safety threshold levels are then used for the next calculation. For our performance the figures for odour and safety levels are: ammonia, odour 6ppm, safety 20 ppm (or 14 mg/m3); acetic acid, odour 0.016 ppm, safety 10ppm (25mg/m3); ethyl acetate, odour 0.61 ppm, safety 200ppm (720 mg/m3). (BTW mg/m3 and ppm can be converted here.)

Exposure risk calculation.

How we calculate how much we can expose the audience to is calculated using the following ‘parameters’

1)    Exposure level in mg/m3: for our purposes taken as the lowest limit under UK WEL levels (normally this will be the long term exposure levels).

2)    Room size: which for the CCA 5 room is estimated at 540 m3 (14 m x 7 m x 6m with a 7 x 2 x 3 sound balcony cutout). This will be quartered and so it assumes that any odorant will diffuse to the first and lowest quarter around the chemistry station, so we end up with 140 m3

3)    Room extraction. Air change figures will be used if they are available.

4)    A 10 fold safety factor will be used. This is standard value to account for individual variations and comes from the sort of equations used in some of the pharmaceutical guidelines (for example the ICH residual solvents guideline).

The equation we are using is this one:

Maximum odourant mass (mg) = (A x B x C)/D

…where A is the room volume, B is the exposure level, C is the dilution factor induced by the number of air changes over the exposure duration (= number of air changes +1/1) and D is the safety factor.

Factoring these all in we get the following limits for the total quantities of these chemical used for the duration of the performance: Ammonia 200 mg; ethyl acetate 10 grams; acetic acid 350 mg.

Those of you will have noticed that the ratio between the odour and safety threshold of ammonia is far less than the 10 fold safety factor, so theoretically nobody will be able to smell the ammonia! However we’ll just have to see what happens!

Sample handling

The ammonia and acetic acid are mixed 1:3 with ethanol. This means they are safer to handle accurately dispense. I’m only going to have ‘on-stage’ as much material as I’m theoretically allowed, and so this minimises the amount of material that could be involved in a spill. Even so, if spilt on skin or in the eyes this could be nasty, so the operator will have gloves and eye protection on. The odourants are dispersed by being placed dropwise on a petri dish on a hotplate heated to around 50˚C. A fan blows air over the petri dish to held the solvents evaporate. The advantage here is that nothing is dispersed until its need, and we can quickly remove and cover the petri dish if we suddenly need to stop. dropped onto. A drop of ethanol from a glass pasteur pipette weighs 12mg on average

Final thoughts

We have also checked that the rooms air conditioning does not recirculate are to other parts of the facility – this is very important!!

And there are different ways of setting these limits. For example the ICH guidelines suggests that you could expose an individual to 50 mg for acetic and ethyl acetate. So for an audience of, let say, 100, that 5 grams. Interestingly the acetic acid level we’ll be working with is 10x less than that value, while the ethyl acetate is about 3 time higher.


2 responses to “This lab smells….

  1. Pingback: Cryptic Science | Experiment, Experience, Explore and Engage·

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