Why unknown hazards account for the poor health of the children

From the text:

Their children were not as well as they had been at home; but how could they know that there was no sewer to their house, and that the drainage of fifteen years was in a cesspool under it? How could they know that the pale-blue milk that they bought around the corner was watered, and doctored with formaldehyde besides? When the children were not well at home, Teta Elzbieta would gather herbs and cure them; now she was obliged to go to the drugstore and buy extracts--and how was she to know that they were all adulterated? How could they find out that their tea and coffee, their sugar and flour, had been doctored; that their canned peas had been colored with copper salts, and their fruit jams with aniline dyes? And even if they had known it, what good would it have done them, since there was no place within miles of them where any other sort was to be had?