Here's why:

* Color: Nitrogen dioxide has a distinctive reddish-brown color, especially in higher concentrations.

* Smell: It has a pungent, irritating odor.

* Toxicity: Nitrogen dioxide is a toxic gas that can be harmful to human health.

However, it's important to note that other gases can also appear reddish-brown depending on their concentration and lighting conditions. For example, bromine vapor can also have a reddish-brown color.

If you encounter a reddish-brown gas, it's crucial to take precautions and identify the source. It's best to consult with a professional to ensure your safety.