Assuming 100nSv per scan (first comment in this post). 10s per scan. ~4000 scans in 12h.
-> 400uSv = 0.4mSv.
Background where I live: 2mSv per year. So this is a fifth of the yearly background dose at my place. But in a single day.
For example, scan the entrance/exit of a parking garage (such as one under supervision) regularly, and you have a realistic chance of severely harming the guy at the ticket booth.
I skipped an order of magnitude here. Your calculation seems correct, so we're up to about a mamogram / day (still ignoring the distance and shielding effects of the building, but we're computing the reasonable upper bound on exposure).
Over the year this 0.4 mSv adds up to about 146 mSv. Per the Handy Chart, it's between EPA dose limits for emergency workers protecting valuable property and for those in lifesaving operations. It's about 1.5 times the "lowest dose clearly linked to increased cancer risk".
So in the worst reasonable case, this seems to be indeed causing an unnecessary increase of cancer risk. Personally I think the conditions for that upper bound are very extreme so the real dose in actual use would not be nowhere near the amount calculated above, but at this point I have to agree there's a reason to be concerned and demand more information about the deployment and capabilities (especially radiation output in failure modes) of those machines.
In short: most likely still not in any way dangerous, but the revised margins are much slimmer, therefore there is a reason for concern.
I apologize for misleading anyone by accidentally skipping a zero.
Assuming 100nSv per scan (first comment in this post). 10s per scan. ~4000 scans in 12h.
-> 400uSv = 0.4mSv.
Background where I live: 2mSv per year. So this is a fifth of the yearly background dose at my place. But in a single day.
For example, scan the entrance/exit of a parking garage (such as one under supervision) regularly, and you have a realistic chance of severely harming the guy at the ticket booth.