Scuba cylinders are filled using a compressor designed for scuba. The compressor has a filter that reduce the potential for contaminated air. Two potential contaminants are carbon monoxide and oil vapor. They can cause serious injury if inhaled, especially underwater. Trace contaminants may produce mild or no symptoms on the surface. But as pressure increases at depth, so will the partial pressure of the contaminated gas.
You may be able to taste or smell the contaminants. However, some are odorless and tasteless. Common symptoms of contaminated air are dizziness, headaches, and nausea. If a diver breathes in a lot of carbon monoxide, he or she may have lips and fingernail beds that are bright red or pink. In severe cases, seek medical attention.
Instances of getting contaminated gas are rare. With regular filter changes, it is highly unlikely. But always purchase your air fills from reputable and professional dive stores.