You can't filter out bacteria with cotton.
I think you should be asking yourself "Is the risk, injecting an unsanitary solution contaminated with blood, worth the reward... which you stated is "a little heroin."
Always weigh the risk and potential consequences with the reward/potential gain. This may be overly simplifying it, but you wouldn't run 5 miles in 100 degree weather risking heat stroke for a nickel. Try and apply the same rational to heroin As we begin to find ourselves farther and farther down the road of drug abuse and eventually addiction we put our drug of hoie higher and higher on our priority list. Slowly but surely drugs become more important than eating out because you'd rather buy dope than Chinesefood, then they're more important than time with friends because you'd rather spend the time getting high, etc etc. But you have to draw the line somewhere... and your health is not something you want to be gambling with.
I'm not trying to sound preachy and I apologize if it comes off that way. But I really think that if you're willing to IV what's most likely an unsanitary, bacteria-filled solution just for the little bit of heroin (that probably isn't enough to get do much of anything) then I think you need to take a good hard look at yourself and your drug habit. Would this risk have been acceptable to you before you began using heroin? And even if there's enough for you to feel it a little, is that short period of time spent high worth any permanent damage to your body? Less than a dime of heroin is meaningless in the scheme of things unless of course the desire to do said amount of heroin results in a consequence you have to carry for the rest of your life.
I'm an IV heroin user and I would absolutely not inject that. Even if it wasn't contaminated, a tiny little bit of heroin isn't even worth doing unless you're just taking whatever you can get to prevent dope sickness.
Also, I don't know where you're located but I believe in some areas if left a long time heroin will convert back to morphine. Especially if exposed to moisture. Anyone remember the source for this?