Why would my claim be denied under the PACT Act for health problems associated with being in Kandahar downwind from the burn pit?

A presumptive claim has three parts - Time, Location and Health Condition. If you meet the time and location parts (it sounds like you do), then it's possible that you don't meet the health condition. Read your denial letter - it should tell you why they didn't accept your claim.

Double check the list of conditions at the PACT Act page* that lists 'burn pit and other toxic exposure conditions" that are now presumptive.

If your condition DOES fall into those categories, then it's possible that the evidence you submitted didn't clearly show the condition or (and I've seen this happen because somehow a DD214 didn't get entered into the system) that you were in the required time/location.

Double check and talk to the Service Officer at the VSO you've been working with (if you haven't been working with a VSO, then you should). If there is an absolute mistake on the VA's part (such as they didn't enter evidence you submitted into the system for determination), then have them submit the missing evidence (that was already submitted..) and do a "CUE Claim" (CUE stands for Clear and unmistakable error" - that will go a LOT faster than if you appeal the decision.

Again though, this is only if there is a clear error on the VA's part .. if they didn't have enough evidence submitted the first time you'll have to go through the appeals process.
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* https://www.va.gov/resources/the-pact-act-and-your-va-benefits/

Recommend you sit down with Service Officer from either (your) County Veterans Service Office - or, seek out one from a nearby VSO - Veteran's Service Organization such as VFW, American Legion, or AMVETS -- bring the VA letter denying your claim with you and your original claim submission.

When was the claim submitted? What condition was the claim for? Exposure and proving you were there is only a piece of the puzzle. It also has to be for a specific condition

I came back from Kuwait/Iraq Feb 03-Feb04 with a constant cough. I was around the burn pits in Kuwait, sand storms, burn pits in Balad, burned sh!t, and like you VA said was not the problem. Yet there were soldiers who came back with the same thing I did, but the difference is a lung biopsy was done on them and found they had taken in heavy metals into their lungs from the dust. A biopsy was the only way of finding it. No biopsy was ever done by the VA on me, so I have a current appeal on fiile. My problem now is I have no way of checking on my claim as I have been locked out due to the new security protocols. they can not verify who I am because the DS Login -DMDC which I have always used will not accept a Danish phone number so they can verify who I am. A flaw in the system they have known about since it was launched last fall. So I am not surprised the VA has problems. I am in process of contacting the Chairman of the Senate Veterans Committee to get something done.