And if the fill plant doesn't vacuum out that potential contamination, particularly the most probably acetylene, before putting high pressure pure O2 on top of it, their fill plant will go BOOM! Any traces small enough to not be a safety problem at the fill plant are also too small to be a safety problem breathing the O2. If acetylene was toxic enough to harm you at a tiny fraction of a percent, welders would be dropping dead all over the place just from turning on the acetylene to light their torches.