One of the most misleading distortions being floated by political opponents of the Iran nuclear deal is the “24-day” loophole meme: Iran would be able to hide all evidence of any nefarious nuclear weapons work during the 24 days it may take inspectors to gain access to a suspicious site.

For starters, International Atomic Energy Agency inspectors would have continuous daily access to all Iran’s declared nuclear facilities. If Iran does not allow anytime inspections of any declared site, it could result in the reimposition — or “snapback” — of sanctions.

The 24-day rule applies only to undeclared suspect sites anywhere in the country. Because inspections anywhere at any time can be complicated to work out, a procedure was devised to address the problem.

Why 24 days? Iran and the atomic energy agency first would have a maximum of 14 days to come to an understanding about how to carry out the new inspections. In the absence of an agreement, the members of the Joint Commission – the United States, Britain, France, Germany, China, Russia and Iran — must resolve the issue, by consensus or a vote, within seven days. Tehran would have three days to implement the decision. So, the 24 days is not a gift to Tehran that would allow it to hide potential nuclear malfeasance — it is just the maximum period allowed to hammer out a way to inspect any undeclared suspicious facility in Iran.

More important, critics insist, Iran could use those 24 days to hide evidence of nuclear materials. This is not going to happen. It would be virtually impossible even in 240 days, let alone 24. Even a nanogram, or one-billionth of a gram, of leftover dust from nuclear-weapons related work — such as covert enrichment at a suspect site — could be detectable.

The main way the agency could find incriminating dust is with a “swipe sample” using a super-clean cotton cloth. The wipe would be applied to surfaces, especially where dust naturally collects, including corners of a room, cracks, bolt holes, equipment interiors and where walls meet the floor.

As the agency itself states: “Any nuclear process … will also produce particulate materials with particle dimensions in the 0.1 [to] 10 micrometer range. Such small particles are believed to be quite mobile and will travel several meters from their point of origin due to air currents or human activity. This mobility also makes it extremely difficult to clean up an area to such an extent that no particles remain available for swipe sampling.” [emphasis added]

The swipe samples from a suspect site would be taken to a laboratory, where the atomic energy agency can use a variety of highly sensitive methods to pick up any infinitesimal incriminating nuclear particles. Isotopic ratios, chemical forms or particle shapes can all provide clues about where the nuclear material came from and how it was produced.

One particularly powerful method is known as “fission track-secondary ion mass spectrometry.” Particles from a swipe sample are irradiated with neutrons on a Lexan (plastic) plate. If there are fissile materials in the sample, they would become unstable and split apart; heavy fission-product particles would be produced. In sufficient quantities, these particles would leave tracks in the plate that can be viewed by acid etching.

The lab scientist can then focus on suspect particles and examine them further with mass spectroscopy, a powerful way to identify particles based on their charge-to-mass ratio. The process is painstaking and time consuming, but it is a highly sensitive way of identifying suspect particles.

The bottom line is that it is almost impossible to get away with messing around with nuclear materials. Nuclear fingerprints are not removable.

“You cannot get rid of them by cleaning,” Stephan Vogt, head of the atomic agency’s Environmental Sample Laboratory told Reuters in 2013. “You cannot dilute them to the extent that we will not be able to pick them up. It is just a matter of time,” he stated, before the atomic energy agency detects any incriminating residue.

In fact, one potential problem with environmental sampling is that it can often be too sensitive. Because samples are frequently collected by nuclear inspectors who are working in contaminated environments much of the time, their clothing, equipment and even their hair can introduce false positives from different locations.

Such cross-contamination may have occurred in inspections of a bombed Syrian site while Olli Heinonen was head of the safeguards department. The result was a botched analysis and invalid conclusions.

To address the possibility of cross-contamination and false positives, the atomic energy agency has devised strict procedures that require two inspectors, sterile conditions, background and control samples and procedural training. Particle samples are also supposed to be processed in multiple laboratories in different countries under a double-blind procedure to preclude any tampering with results — or the planting of evidence. This was not done in the Syria investigation.

If the procedures are followed properly, the conclusions are considered highly trustworthy.

It is imperative that official IAEA sampling procedures are adhered to scrupulously, and that the integrity of inspectors is beyond reproach. If false accusations are made based on faulty procedures, it would cast doubt on the entire international monitoring system. This is why the atomic energy agency must be extremely careful that technical mishandling of the samples, as reportedly occurred during the Syria investigation, is not repeated in Iran.

Another important consequence of the exquisite sensitivity of IAEA’s particulate- detection methods is that it is virtually impossible to hide traces of nuclear material even outside the site, for example, by partial paving of a site. Some analysts have alleged that the paving work at Iran’s Parchin military complex could sanitize it. But this is incorrect. Environmental sampling is still possible where large parts of a site remain undisturbed. As Robert Kelley, a former IAEA inspector, explained about the Parchin site: “The fact that the building’s immediate vicinity has been largely untouched on the west side strongly suggests that the purpose of the earth-moving operations was for construction and renovation work and not for ‘sanitizing’ the site by covering up contamination.”

In addition, the preferred method of sampling at the Parchin site would be to take sample swipes from inside the buildings of interest. It matters little what construction Iran does outside. If outdoor sampling were needed for some reason, it could be gathered from the large undisturbed area west of the building of interest.

Twenty-nine top U.S. scientists — including Nobel Prize winners, senior experts in arms control and former White House science advisers – wrote to President Barack Obama this past weekend to praise the Iran deal. They called it “technically sound, stringent and innovative.” Instead of listening to the complaints about the 24-day meme, Congress should pay heed to these experts.