Which characteristic describes highly toxic substances?

Highly toxic substances are defined by their potential to cause severe health effects or even death, often at very low exposure levels. This characteristic highlights the danger these substances pose, where even minimal contact, inhalation, or ingestion can lead to serious health consequences. Understanding this is crucial for safety protocols in environments where such substances are handled.

While some substances may be harmful primarily in high concentrations or might be managed with proper precautions, highly toxic substances defy these norms by being dangerous at lower levels. Additionally, the physical properties of a substance, such as being colorless or odorless, do not inherently determine its toxicity. Therefore, the defining characteristic is the severe risk to health that makes certain substances highly toxic.