Poppies (Symbol)
One reason that this poem can get away with being so short, and yet so emotionally evocative, is because the poppy comes with its own preexisting symbolism. Poppies traditionally stand for sleep and death, in part because they are the source of opiates and thus can produce a sedative effect. Especially in the twentieth century, the flower was also used to commemorate soldiers who had died at war. However, Plath layers more symbolism into the image of the poppy: in this poem, it becomes a symbol of unexpected natural beauty, catching the speaker's attention in a dull urban environment and prompting additional attention to various other sources of unexpected beauty. Furthermore, the poppy comes to represent the inextricable relationship between birth and death.
Carbon Monoxide (Symbol)
The speaker notes the contrast between the vibrant poppies and the urban sky, choked with carbon monoxide. It's only intuitive that carbon monoxide would symbolize something unpleasant. After all, it's a dangerous, poisonous gas. Here, it symbolizes industrialization, and all of the alienating urban life that follows it. The scientific-sounding words "carbon monoxide" themselves prompt a feeling of detachment, mimicking the industrialized detachment from nature and feeling that carbon monoxide symbolizes.
