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 ambient pressure increases at depth, so does the partial pressure of the contaminants in the breathing gas.
You may be able to taste or smell certain contaminants. However, many 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. If you suspect that you have contaminated gas, DO NOT DIVE. In severe cases, and certainly when signs and symptoms persist, seek medical attention.
Instances of getting contaminated gas are rare. With regular compressor service and filter changes, it is highly unlikely. But always purchase your air fills from reputable and professional dive stores.