Just to add to question 1/ above, how long can you stay in the red for? briefly? can you hold in it? surely accumulative damage starts to play a part..
Not 100% sure but the red coloured area ( UK MET Office ) will mostlikely illustrate the so called Zone 2 while the black coloured is the Zone 1.
Zone 1: Limited No-Fly Zone is an Area with a High Density Volcanic Ash Contamination. A "No fly zone" - which includes the main area/core of the volcanic fallout, with an additional buffer zone.
Zone 2: Potential Contamination Zone is an Area with a Low Density Volcanic Ash Contamination. An area outside Zone 1 where flying can be conducted when actual conditions, risk assessment and test(s) can establish, that flights can be conducted at an acceptable level of safety and requires prior permission from the operators Authority.
At least Icelandic AOC holders have to apply for such permission from the ICAA. Prior such an application you have to set up several operational & maintenance related procedures ( e.g. recommendations from the TC holders ). In addition to the daily checks i.a.w. EASA SIB No: 2010-17, the ICAA for example requires, when an aircraft has been operated for a maximum time of three accumulated flight hours in "Low density Ash" (Zone 2), an inspection in accordance with aircraft and engine manufacturer guidelines for inspection after flying in volcanic ash shall be performed.
Not sure how other authorities are handling that and if they have these accumulated 3 hours restriction in place as well.
However, GE for example says for Zone 2 ( greater than 2E-4 grams/meter cubed, but less than 2E-3 grams/meter cubed predicted zone, "Enhanced Procedures Zone" ): There are no additional maintenance or operational procedures required to ensure safe operation!
RR and PW have a set up a little bit different recommendation.
While the engine OEM's are not calling for any time limitations when flying thru Zone 2, the local authorities are handling that obviously different. So your question can't be answered with a "standard statement".
BTW: the local TV station RUV made a nice video clip last evening from the local at eruption side:
To see the clip, click on "Horfa á myndskeið"!
Currently we have again massive ash fall in the south east and all the glory can be watched here :
Also a good overview in regards to the actual seismicity( Eyjafjallajökull vulcano is at the glacier in the south ) can be found here:
( not sure if those links have been posted already )
Greetings from Iceland
Sunny 