How dangerous is an industrial scale styrene spill?
According to this safety data sheet (SDS), styrene monomer has a Category 3 respiratory classification, where 1 would be most serious. Category 3 is defined "substances produce transient (short duration or temporary) target organ effects." 

Add to that these hazard, precautions and carcinogenicity statements: 


H335: May cause respiratory irritation.
H372: Causes damage to organs through prolonged or repeated exposure if inhaled.
P260 Do not breathe dust/fume/gas/mist/vapor/spray.
NTP Reasonably anticipated to be a human carcinogen


Read the cited SDS or others for statement on environmental precautions and cleanup. Frankly, they're a bit vague to me, "If the product contaminates rivers and lakes or drains inform respective authorities."