This also has an apocalypse setting common to many modern stories: "a closed human habitat has limited oxygen & if you have a third child, someone from family will have to be thrown out ('nomaded')". Only it's far more readable than almost everything I've seen with this setting.
Story summary.A couple of thousands years into future, much of the oxygen has vanished from earth's atmosphere, all large animals except humans are dead, even humans are very few & dwindling, nitric acid pollutes all open water sources, & there are violent nitrate based plants prone to explosion.
People live in "closed" cities, with water-pool based airlocks. There also are some hardy nomads, folks who were thrown out of cities because of overpopulation but survived. You cannot venture outside without breathing mask & oxygen tank.
There are also "natives" or "Invadors" or "Observers" - curious galaxy-traveling aliens well suited to this earth atmosphere.
This is the story of a nomad couple, their 5 year old daughter, & their alien friend, who arrive at a city somewhere near Boston coast with supplies of salvaged glass & copper nuggets produced from ocean by "pseudu-life" that still survives. Only the customers have some deadly ideas & all hell breaks loose soon after their arrival...
Fact sheet.First published: 1980.
Related: Stories of Hal Clement.