I am looking on different scenarios for atomic winter. I think if there is a lot of NOx forming in the atmosphere, chanses to survive are slim to none. You would need a NOx-scrubber with good efficasy and just one breath of the unpurified air would possibly kill you. It does matter offcourse how much NOx that is present but this is toxic. Formation of NOx is now ruled out so maybe its less of a problem. If it is more general smoke from fire in plastic, wood, oil, fabric etc this can be filtered with a HEPA and a activated carbon filter. These are available in good quality air purifiers. So, a room say 40m2 with tight windows and a controlled air supply and a filter could maybe keep you alive. If you change filter regularly and have a stock of these maybe you can survive 2months? 1 year? The numbers I have seen for the time to get good clean air again is 2months or 1-4 years. The air will slowly get better outdoors so filters will be less clogged after some time. How long do you think one can survive under this conditions? Will there be rescue-people coming after one month to help you? How many people are prepping for this situation? The military have resources with filtrated air, the higher ranking officers will survive for long time if they get into these facilities. Also the president etc,