@Pace
I think the permitted level was something equivalent to a thimble full of ash in a volume equivalent to a 4 bedroom house
Possibly the ratio you were referring to was the May 2010 CAA revised limit of 4 mg per cubic metre of air, which equates to aprox 1g of ash per 250 cubic metres, (roughly the equivalent of your 4 bedroomed house example).
To help give this apparently insignificant figure better context, consider that a modern aircraft engine (with 118" fan diameter) has an intake fan area of about 7 m^2. When travelling at 250kts (aprox 125 m/s) this will ingest AT LEAST 875 cubic metres of air per second by virtue of mere 'ram air' effects, without any consideration for the significantly greater volume actually drawn in due to induction when producing thrust.
This is equivalent to ingesting around 4 grammes of ash as a minimum, EACH SECOND, or a WHOLE KILO of the stuff every 4 minutes. The reality is the figure would be considerably more for the reasons given above.
Given the significantly lower melting point of the clogging constituents of the ash than the turbine temperatures, I would certainly not wish to be the subject of live experiments with such quantities of ash in any engine that was keeping my backside from hitting the ground!