Which term best describes a reaction where the combined effect of two or more substances exceeds the sum of their separate effects?

• A. Synergistic
• B. Additive
• C. Complementary
• D. Interactive

Answer: (A)

When two substances interact to produce a combined effect greater than the sum of their individual effects, we call that synergistic. This is important in hazardous materials because mixing or simultaneous exposure can produce a hazard that’s bigger than you’d expect by looking at each substance alone, so you treat the combination as more dangerous and plan correspondingly—more ventilation, stricter containment, or enhanced PPE may be required.

For example, two irritants might each cause mild symptoms separately, but together they can cause severe irritation or systemic effects beyond the simple addition of those symptoms. If the effect were merely additive, you’d expect the total harm to equal the sum of each substance’s harm. If it were just described as complementary or merely interactive, those terms wouldn’t convey the amplified impact you must anticipate.