The word "nonhazardous" implies the absence of risk or danger, but there are several antonyms that convey the opposite meaning. One of the simplest antonyms is "hazardous," which suggests that a substance or situation poses a potential threat to health or safety. Other antonyms include "unsafe," "risky," "perilous," "precarious," "toxic," "dangerous," and "volatile." Each of these words conveys the idea that something is potentially harmful, and should be approached with caution or avoided altogether. Conversely, "safe," "secure," "stable," and "non-toxic" are all synonyms for "nonhazardous" and suggest that something is free from danger or risk.