Caulk: is there anything even semi-natural?

[Gumby]

I went to the hardware store today to check out some caulk, among other things and the ingredients were appalling! I felt bad for even touching the container...formaldehyde being one of them...

so is there anything less icky? Our furnace ducts leak and in an attempt to make our home as efficient as possible we wanted to seal them up. I'm considering duct tape at this point.

Ideas? Does better efficiency trump what's in the tubes?

[Mara232]

Here you go:

http://www.naturalbuilthome.com/prod...adhesives_178/

[talia rose]

clear silicone (the kind used for fish tanks) is considered the safest.

[Gumby]

kewl--thank you!