Symbolism, Imagery, Allegory
Hey, what's that weird cloud of smoke over there? The radio says it's a "feathery plume" (21.29). No, wait. Now they're saying it's a "black billowing cloud" (21.78). Scratch that. Now it's a full-blown "airborne toxic event" (21.128). From the second Jack Gladney first spots the cloud of toxic gas hanging on the horizon, he doesn't really know what to make of it. Nobody does, really. It seems like their brains don't even process what they're looking at. The only way they know what to call the thing is to listen to the radio.
Just like we saw with the most photographed barn in America, the "the airborne toxic event" shows us how much our words tend to shape reality more than the other way around. Nobody knows what they're looking at in the sky until the radio tells them what to call it. Then the radio tells them something else, and that becomes the new reality. This doesn't only happen with the toxic cloud itself, but with the symptoms that the gas (Nyodene D.) is supposed to cause in people.
At first, the radio tells everyone that exposure to Nyodene D. will cause "heart palpitations and a sense of déjà vu" (21.121). But after people start getting déjà vu, the radio changes its message and says that déjà vu isn't a symptom. The fact that everyone has started getting déjà vu, though, shows how the radio's message has already convinced everyone that a certain thing is real.
Through the "airborne toxic event," we see just how much mass media can convince huge amounts of people (even a whole town) to only see what the media tells them to see. Not only that, but it can convince huge amounts of people to feel what the media tells them to feel, even something as nuanced and cerebral as déjà vu.