**Core Concept**
Carbon monoxide (CO) is a colorless, odorless gas that can cause systemic toxicity by binding to hemoglobin, forming carboxyhemoglobin, and reducing oxygen delivery to tissues. This process occurs without causing local irritation, unlike other gases such as ammonia or chlorine gas.

**Why the Correct Answer is Right**
Carbon monoxide has a high affinity for hemoglobin, with a binding constant 210-240 times greater than that of oxygen. This means that even small amounts of CO can rapidly bind to hemoglobin, leading to the formation of carboxyhemoglobin and a decrease in oxygen delivery to tissues. The lack of local irritation is due to the fact that CO does not trigger the release of chemical mediators or cause damage to the respiratory epithelium.

**Why Each Wrong Option is Incorrect**
- **Option A:** Nitrous oxide is a gas that can cause local irritation, particularly at high concentrations, due to its ability to stimulate the release of chemical mediators.
- **Option B:** Ozone is a highly reactive gas that causes local irritation and damage to the respiratory epithelium, leading to symptoms such as coughing and shortness of breath.
- **Option C:** Ammonia is a gas that causes severe local irritation and can lead to respiratory failure, as well as systemic toxicity due to its ability to damage the respiratory epithelium and stimulate the release of chemical mediators.

**Clinical Pearl / High-Yield Fact**
Carbon monoxide poisoning can be difficult to diagnose, as patients may present with nonspecific symptoms such as headache, dizziness, and nausea. A key diagnostic clue is the presence of carboxyhemoglobin in the blood, which can be detected using a co-oximeter.

**Correct Answer:** C.