Because our narrator is omniscient and knows all, he can show us Jurgis's miserable thoughts and feelings about the horrors of Packingtown. Because the purpose of The Jungle is to persuade us that Packingtown is disgusting and should be changed, the narrator indulges in long, intense descriptions of Jurgis's pain at the injustices he experiences. Take, for example, this extract:
Ah, God, the horror of it, the monstrous, hideous, demoniacal wickedness of it! He and his family, helpless women and children, struggling to live, ignorant and defenceless and forlorn as they were – and the enemies that had been lurking for them, crouching upon their trail and thirsting for their blood! That first lying circular, that smooth-tongued, slippery agent! That trap of the extra payments, the interest, and all the other charges that they had not the means to pay, and would never have attempted to pay! (18.54)
The "it" in question in the first sentence is the financial exploitation Jurgis's family has suffered. In this scene, Jurgis is sitting in jail after attacking Connor. He is worrying about how his family will cope with their monthly house payments while he, the primary wage earner, is rotting in a cell. He exclaims about "the monstrous, hideous, demoniacal wickedness" of the high house prices he has to pay. He worries about his family – "helpless women and children" – and their pathetic inability to handle the challenges facing them to make ends meet in Packingtown.
We're calling the tone of these passages oppressive because they are meant to emphasize the inescapable despair of the social problems facing men like Jurgis. The prose is also intense because it uses so many emotionally charged terms: for example, describing the businessmen who have given Jurgis and his family unfair deals as "enemies lurking for them" and "thirsting" for his family's "blood" is highly dramatic. This text is also unsettling because Jurgis feels such a profound sense of helplessness and desperation in managing his own life. As a result, his house purchase seems to be a "trap" brought about by a "lying" advertisement and a "smooth-tongued, slippery agent." Jurgis has no power over his own life and choices because he and his family have been deceived left and right. The reader is supposed to be upset at Jurgis's plight, so the tone of the writing is intentionally unsettling.
Still, you might say that of course the narration that examines Jurgis's perspective on his life will be oppressive, intense, and unsettling. Jurgis's point of view is bound to be biased and miserable. Yet, even the more journalistic moments in the novel take on a similarly emotional tone. Consider one of the first passages describing the poor working conditions for men in the fertilizer and lard processing rooms of the slaughterhouse:
These people could not be shown to the visitor – for the odor of the fertilizer man would scare any ordinary visitor at a hundred yards, and as for the other men, who worked in tank rooms full of steam, and in some of which there were open vats near the level of the floor, their peculiar trouble was that they fell into the vats; and when they were fished out, there was never enough of them left to be worth exhibiting – sometimes they would be overlooked for days, till all but the bones of them had gone out to the world as Durham's Pure Leaf Lard! (9.11)
The narrator is describing which parts of the factory are off-limits to visitors because they are would be too upsetting for the general public to see. What makes these rooms bad for PR is 1) the smell, and 2) the "open vats near the level of the floor," which are full of strong chemicals for processing animal fat. If a worker falls into one of these vats, his body will get dissolved down to the bone. If the accident isn't noticed in time, his fat will join the cattle fat used to make lard. Obviously, this is extremely disturbing information. The exclamation point at the end of the passage scarcely seems sufficient to underline the intense horror of this news. The tone of the whole passage is meant to unsettle the reader because the reader should be unsettled to discover what might be going into his or her store-bought lard.