I live in this bungalow,for 17 years, all my exterior walls and some ceilings have a black residue all over but highlighting(darker) the drywall nails, screws and where any drywall patches were done. I've wiped one area on the wall and the black residue comes off.Can't see the nails anymore. The humidty in the house is 37% and the heating is gas. This started 2 years ago with one wall in one room and has now spread to the rest of the house. The interior walls and furniture are OK, no black residue or nails showing through. Nobody I've asked had an answer for me.?

Your humidity is too high and that residue is mold. Mold spores are blowig all around and keep soreading. Most of your excess humidity condenses on the nail heads because they are cooler. Lower the humidity, wash all the walls with mildewcide, paint with fresh paint containing a mildew preventer and change you furance filter once a month for the next year

Humidity is 37%, I was told that this is not high but normal. There are no signs of huidity, wet windows, as I said the interior walls have no black residue, why is there no mould there?? My furnace filter does get changed monthly.
I'm not convinced it's mould. Thanks for your reply.

Your humidity isn't 37% but your RELATIVE humnidity is 37%. You need to compare it to the current temp using a hygromoeter which most folks don't have available. The reason I your interior walls aren't getting mold is they are warmer than the exterior walls and ceiling. Take a mister type spray bottle and mix 1 part bleach to 4 parts water, spray one brief blast on the residue. What color does it turn, brownish I'm guessing

The reading of 37% was taken with a Hygromoeter. I tried spraying the bleach/water mixture on those areas and the colour remained the same, did not turn brown or any other colours. I wiped the area down and the paper towel had a black residue on it.

Molds are extremely dangerous. If I were you, invest on removing that drywall and whatever insulation you may have. I'm guessing there is a water infiltration behind the wall. Once you remove the drywall inspect for water infiltration and clean the studs with bleach (wear a mask... molds will affect your lungs) Once satisfien that there are no leaks and that you removed al the molds, apply a vapour barrier and re-install fresh drywall.

Trust me... you do not want to this half way.

Here is a good link to help you out.

http://www.betterhealthusa.com/public/183.cfm

rioval tested for mold and it appears this is not mold.

That is very odd... Thanks ballengerb1

I though so too, I was sure it was mold until it was sprayed with bleach and stayed black. I was 99.999% sure