Now, I know about air pollution. For a start, "micrograms" is not a sensible unit for air quality. If you don't specify the unit of air used, the figure makes no sense - was that 996 micrograms in a small volume of air, or in the entire station? Also, you have to state over what period of time the readings were taken. Usually, particulates are measured in micrograms per metre cubed, and the readings taken averaged over 24 hours to provide an instantaneous snapshot.
If the quote of "70 times the amount of dust found in an average office" is correct, then the unit of the 996 is indeed micrograms per metre cubed. And this makes no sense. At a level of 100 micrograms per metre cubed or more, someone like me with endemic asthma would be in considerable discomfort. It's as heavy as the pollution level immediately adjacent to a large motorway.
I can't believe those figures are right. Even 99.6 micrograms per metre cubed would seem too much. 100 micrograms per metre cubed of particulates is enough that you can smell and taste it in the air. Remember the heavy smoke smell that hangs in the air for a couple of days around Bonfire Night? That's what a level of 100 micrograms per metre cubed feels like. And 996 micrograms per metre cubed! That's more polluted than the most serious pollution episode that I studied in my PhD. If the Tube was really that bad, asthmatics would be dying every time they tried to use it.
I want to see the actual report for myself, so I can find out what the figures really were. My belief is that the scientists found a level of 996 micrograms per metre cubed in 24 hours. Not an average level - a total level. This would make sense, as it would average down to 41.5 micrograms per metre cubed in one hour. I could believe this to be the level of dust pollution on the Tube. It's high enough to cause health problems if you were to be exposed to it continually, but not so high that someone with medium-grade asthma would notice it on a single journey. It would be important enough to issue a report on it, as it is sailing close to the WHO safe limit, but it would only be actively dangerous for Underground workers who spent 12 hours a day out in the tunnels without the protection of a face mask. Not noticeably more dangerous for commuters than simply standing in a London street.