A little good news and a little bad news, perhaps, for Froome. The good news:
In the past, Wada has not adjusted the salbutamol threshold in test results to account for "high urine density", which can increase when you are dehydrated.
But that is changing from 1 March 2018, and a Wada spokesperson told BBC Sport that for any case currently being adjudicated, "the most beneficial rule to the athlete would apply".
http://www.bbc.com/sport/cycling/42417297
As I pointed out upthread, he would still have to be severely dehydrated (commonly defined as urine SG > 1.030) for the correction to lower his salbutamol level below the threshold, and my understanding is that if his urine were that concentrated, the sample would not have been approved for assay. In fact, his USG would have to be 1.040 to lower his level to the threshold of 1000 ng/ml, or 1.033 to reach the decision level of 1200 ng/ml. So I wanted to get an idea of how often riders are dehydrated to that extent.
The first two studies are by the same group (led by one L.E. Armstrong, no less), and involved an analysis of men and women riders in a 164 km ride, not race, under intense heat, 35 – 39o C (it's called the Hotter N' Hell Hundred). In the first study, they pooled data from three years. There were 88 men, with a mean USG of 1.019 +/.007 before the ride, and 1.024 +/- .007 after the ride. Based on these values, there would be about 16% with values > 1.030, and about 2% > 1.040.
http://sci-hub.la/10.1123/ijsnem.2015-0188
This is roughly consistent with another study by the same group of 42 men from that ride, reporting that 11 had USG > 1.030, and none > 1.040.
http://sci-hub.la/10.1519/JSC.0b013e318240f677
In another study by a different group, 11 males performed 90 minutes at 50% V02 max, followed by a self-paced 20 km TT at 35o C. Various hydrated or dehydrated states were tested. The USG values were all relatively low, generally about 1.010 at start and 1.015-1.020 at end, with SDs ranging from .002 - .008.
http://sci-hub.la/10.1111/sms.12343
Still another study analyzed 37 male mountain bikers over a 120 km Swiss Masters race. USG pre-race was 1.010 +/- .007, post-race was 1.014 +/- 0.007. They found that intake of 700 ml of fluid per hour was enough to prevent dehydration. However, temperatures were relatively cool, ranging from 5 – 11o C, the lowest temperature at altitude, > 2000 m.
http://sci-hub.la/10.1097/JSM.0b013e3181b47c93
Another study examined 18 mountain bikers in a 3 stage race. The highest mean value of USG was after stage 3, 1.025 +/ .001. Temperatures ranged from 9 – 22o C.
http://sci-hub.la/10.1136/bjsm.2008.049551
So overall, at moderate temperatures, dehydration does not seem to be a problem, while at very high temperatures it could be. I'm not sure how hot it was for Stage 18 in the Vuelta? In any case, the studies at high heat on the road were of amateur riders, who may not have been as careful about hydration as pro racers would be, though the men did take in an average of 5-6 l. of water over the entire ride, about 9 hours.
And a little bit of bad news, if Froome thought that he could get off without showing that 2000 ng/ml is possible under some conditions. I finally tracked down some information on Piepoli, who tested positive for salbutamol in the same Giro (I believe the same stage) that Petacchi did. His urine level was 1800 ng/ml., higher than Petacchi’s. Both riders got off initially through decisions by their national federations, but the Italian Olympic Committee (CONI) appealed Petacchi’s decision to CAS, where he lost (I discussed this CAS decision upthread). Piepoli held a license in Monaco, and his decision was not subject to appeal. He also claimed he had a medical certificate that allowed him to take doses of the drug that could result in urine levels > 1000 ng/ml. I’m not sure if that’s the same as a TUE, which at that time was needed just to be allowed to have levels up to 1000 ng/ml, but apparently it was good enough for Monaco. A Barcelona lab attempted to determine if his salbutamol was inhaled or taken systemically, but couldn’t reach a definitive conclusion.
AFAIK, Piepoli is the only rider in the past decade who tested for salbutamol > 1000 ng/ml. and wasn’t sanctioned, but it’s pretty clear from his case that he had no better argument than any of the riders who were sanctioned. If his case had gone to CAS, he almost definitely would have lost just as Petacchi did. Though the panel might have accepted that he took too much by mistake, as they did with Petacchi, as asthmatics have commented on this thread, taking doses of the required amounts by inhalation would be extremely unlikely. Also, about a year after the salbutamol positives, Piepoli tested positive for CERA. So I don’t think his case has much relevance for Froome’s. If Froome can't show that taking salbutamol within the allowed amounts can result in the high level reported, either as a result of dehydration or some other factor, there is no precedent that should allow him to avoid a suspension, and I would think of at least nine months.
http://www.cyclingnews.com/news/innocently-guilty-the-petacchi-case/
http://autobus.cyclingnews.com/news.php?id=news/2007/aug07/aug06news2
http://autobus.cyclingnews.com/news.php?id=news/2007/jun07/jun13news2