Final answer:
Radioactive krypton and xenon, due to their noble gas properties, and nitrogen, due to their prevalence in the atmosphere, are gases that can be used for leak checking that cause minimal environmental impact.
Step-by-step explanation:
When performing a leak check on a unit that contains no refrigerant, it is crucial to use gases that cause minimal environmental damage. The use of noble gases such as radioactive krypton and xenon, which are not incorporated into the food chain, may be considered among the safest in terms of environmental impact. These noble gases disperse quickly in the atmosphere and have been deemed tolerable even when released during nuclear incidents because they do not produce highly radioactive zones and are not biologically active. Additionally, nitrogen, a natural component of the air, can also be used effectively for leak testing and has negligible impact on the environment.