Richard
Your operation may be a couple of hours per day but how many years have they been running a couple of hours a day for? When did they last clean up? You are probably the first Manager they have had that cared! The chances are that its tyre dust, if you get an analysis done it will come back as carbon whether its tyres or exhaust. From the tests you have done your engines are running lean so it wont be an exhaust problem. Black tyres are made from a mixture of natural rubber, synthetic rubber and carbon black, the reason carbon is used is because its very hard and the tyres last longer (thats why they make diamonds with it too!). White tyres are made from chalk which is soft, they last about 5 minutes and cost quite a bit more - you end uo with white dust - prettier but much more of it! You may however be using cheap black tyres which contain a lot of carbon - they are too hard and dont last that long. Good tyres to try if you can get them are Gumasol or Marangoni. If you can't get these ask your supplier what the rubber compound used in any replacement tyres you buy is - whats the chemical composition? Ask about the characteristics - grip, hardness, softness. There are rating factors for all these characteristics - a long time ago I used to know the names for them! A simpler rule, however, is dont buy the cheapest.

You will find the fine black dust in every warehouse that uses fork lift trucks. Usually caused by the erosion of tyres

I agree with oldmanforklift and check your air make up units. A lot of dirt can be pulled in from the outside, black or any other color.

Have you sent the black dust for testing to see what material it is?

Check the air handling units. A belt set on a fan will produce a lot of black dust. Are the filters in good shape. Make up air from outside may be laden with dust and if not filtered will be distributed thruout bldg. Outside contamination wind born thru open dock doors. I was reciently in a building that had 2 doors open on bldg. Dust storm outside put.25 in dust over 10000sq ft of wharehouse in about 2 hours. Took customer 3 weeks to clean up. What was stored in bldg before? How long has contamination been there? Over head steel may be laden with dirt and it is slowly falling to floor.

Try wrapping the tires with duct tape...a few of our customers do this when we don't have any units with non-marks available.

The paper you mention will probably get torn up. I would just rent a rider scrubber (make sure they show you how to use it properly - otherwise it is a waste of money) and clean the aisles. They don't come cheap - may rent for $500+ a week. Most rider scrubbers will clean 60,000 sq. ft. in a couple hours with the correct chemical. Find a local scrubber company - they may demo a unit that won't cost you much of anything (except selling you on a walkbehind to keep on hand). Tennant and PowerBoss are a couple in the industry.

Thanks for the thoughts.
We store crates with trade show booths - so no chemicals or other materials for production.
The guys are very careful and don't peel out, etc.

We have a Nissan with smooth tires and a larger Toyota with some tread to them.
We are going to have our HVAC contractor come in and do an air test which will include particulate counts as well. This should help us narrow it down to something from an exhaust (trucks or heaters) or something that is getting blown in.... Seems to be dirty all year round which should rule out the heaters as they just run when it's cold.

We did put a white piece of paper up to the exhaust of each and the larger Toyota did put out some black onto the paper - but with as little as I saw for that short of a time frame, it's hard to believe that it could cause this much dust.

I plan to sweep an area real well and then apply a large sheet of paper to the floor - the leave it for a week - this should give me a good idea of how quickly it's building up.

Any other suggestions or thoughts are appreciated.
Thanks

It is very doubtful that your forklifts are leaving a substantial amout of "black dust". Espically condidering the limited usage you mention above.

The dust probably comes from a product you are storing. For example, Carbon Black which is used in may injection molding operations will make a warehouse filthy.

im assuming your running cusions (solid tires)
are your tires black rubber? if so that may be the source of the dust
the tires wear down like pencil erasers from dead steers and other reasons
the radiator fans and exhaust could be kicking up the shavings
you can try using non-marking (white) tires that prob run about 900 bucks per machine for 5000 lb machines
also check to see if the operators are braking hard (skidding) and if they are pulling holeshots (peel out, etc) when changing directions (slow safe operation may cut down the dust)
dont forget that if this is the source and you go with nonmarking tires you will have to add a static strap or a small length of chain to the frame that drags on the ground
this discharges static buildup (unless u like little shocks and blown ECU's) as the black tires ground static thru themselves
also check the floor condition (large cracks potholes rough areas etc)