when someone shares iv drug works...blood is drawn up into the syringe before the drug is injected. therefore, blood is trapped in an airtight compartment until it is injected by the next person.
what you have described is not a risk for contracting hiv. once exposed to air, hiv becomes inactive and unable to infect.
Only reason I ask is if you can get it from needles can you get it from throns....not stressing really..thought id ask the risk