It's not just toxic, it's incredibly toxic... the threshold for toxicity is below human sensitivity, i.e. if you can smell it, it's already too late. It's also flammable/explosive, poisonous, carcinogenic, catches fire without a spark, has no surface tension so it spreads faster than water, and it's generally the single most dangerous chemical known to man. In the words of Isaac Asimov:

anyone working with rocket fuels is outstandingly mad. I don't mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity.

That's Hydrazine. The other liquid propellant, nitrogen tetroxide, which causes the signature red clouds, is slightly (!) less hazardous. It will still readily kill you. It's odd to see in the twitter photo that there was a fuming red cloud during takeoff, which suggests a problem with the burn ratios. Absolutely zero propellant should be allowed to pass unspent into the air, from engineering, safety and environmental perspectives.