When he wrote "And all be poisoned alike by their summer stenches," he was referring to the terrible sanitation problems that plagued the city's slums. The lack of proper sewage systems, garbage collection, and ventilation led to an accumulation of rotting waste and human excrement, creating a foul, disease-ridden environment that affected everyone, regardless of their social status.
The phrase "poisoned alike" emphasizes the equality in the face of suffering. While the poor endured the worst of these conditions, the wealthier residents, though living in better housing, were still exposed to the same unhealthy environment and the diseases that thrived in it. The stench was a potent symbol of the city's disregard for the health and well-being of its poorest citizens.
By using this powerful imagery, Riis aimed to shock his readers into recognizing the urgency of addressing the issue of poverty and sanitation in New York City. His words served as a call to action, urging the public to acknowledge the plight of the poor and demand better living conditions for all.