Hydrogen chloride, which requires specific conditions in which to form, has been detected in the atmosphere, raising many questions. The Mars we see today is mostly dry, dusty, and barren.

Hydrogen chloride and phosgene can cause symptoms like eye or throat irritation or even respiratory issues like difficulty breathing. "It's always challenging to evaluate the human health risks ...

This then reacts with hydrogen-containing species to produce the hydrogen chloride detected by the Trace Gas Orbiter. The chlorine or hydrochloric acid-rich dust eventually returns to the surface.

The EPA has also screened 291 homes near the crash site and has not detected any levels of vinyl chloride or hydrogen chloride, the agency said. As of Monday, 181 homes still needed to be screened.