What is the nature of ozone (O3) to humans?

Ozone (O3) is considered toxic to humans, which is why it is the correct choice. Ozone can have significant adverse effects on human health, particularly pertaining to the respiratory system. It is a strong oxidizing agent that can cause irritation of the airways, leading to conditions such as coughing, throat irritation, and exacerbation of asthma symptoms. Exposure to elevated levels of ozone can also reduce lung function and increase the frequency of respiratory infections.

While ozone plays a beneficial role in the stratosphere by protecting the Earth from harmful ultraviolet radiation, its presence at ground level as a major component of smog is detrimental to health. The information aligns with environmental health guidelines and studies showing the importance of managing air quality to minimize ozone exposure for the well-being of the public.

In contrast, the other options do not capture the harmful nature of ozone. It is neither beneficial to respiratory health nor neutral, and it is certainly not used as a dietary supplement. Hence, the understanding of ozone's characteristics and its effects on human health firmly supports the identification of it as toxic.