KHANAQIN, Iraq — A 36-hour shamal that swept the Iran-Iraq frontier this week reduced visibility to below 200 meters across returnee transit camps and drove respiratory admissions at frontline clinics to the highest levels recorded since the April 15 ceasefire, aid agencies and Iraqi health authorities said Thursday, complicating an already strained recovery operation along a corridor still managing a cholera outbreak and unfinished mine clearance.

The seasonal dust storm, which Iraq’s meteorological service classified as a category three event on its national five-point scale, blanketed Diyala, Maysan and the Iranian provinces of Ilam and Kermanshah from Tuesday morning until just after dawn Wednesday. Air-quality monitors operated by the Iraqi Ministry of Environment in Khanaqin and Baquba registered PM10 concentrations above 6,400 micrograms per cubic meter at the storm’s peak, more than 70 times the World Health Organization’s 24-hour guideline. Two smaller monitors on the Iranian side, in Mehran and Ilam city, recorded comparable readings.

Doctors Without Borders and the Iraqi Red Crescent reported a combined 1,870 acute respiratory presentations across nine field clinics serving the Mehran-Khanaqin corridor during the storm window, a figure roughly four times the baseline of the preceding two weeks. Forty-one patients required hospital transfer, the majority of them children under five or adults with pre-existing pulmonary conditions, and at least three deaths in transit camps had been provisionally linked to the storm by Thursday afternoon, though officials cautioned that final attribution would take days.

“This was not a freak event — shamals at this time of year are entirely expected — but the population we are caring for has never been less able to absorb one,” said Dr. Mariam Khoury, the medical coordinator for Doctors Without Borders in Diyala, by phone from Baquba. “We have families living in tarpaulin shelters, on damaged ground, with damaged lungs from six weeks of smoke and chemical exposure during the war, and now they have inhaled three days of fine silt on top of that. The body cannot keep stacking insults.”

Officials said the storm had also disrupted the broader returns process. Iraqi border police closed the Mehran and Munthiriya crossings to civilian traffic for most of Tuesday and Wednesday morning, stranding roughly 9,400 returnees at staging points on both sides. Demining teams operating under the United Nations Mine Action Service suspended field work for the full duration of the storm, and route-clearance survey vehicles were grounded by sand intrusion into engine intakes. UNMAS said in a statement Thursday that it expected to lose at least four working days as crews flushed equipment and re-marked task sites where dust had obscured warning tape.

The shamal arrived as humanitarian agencies were still racing to contain the cholera outbreak that the World Health Organization declared in the same corridor on April 26. Updated WHO figures released Wednesday from Cairo put the cumulative case count at 3,940 across the cross-border catchment area, with 58 deaths and a rolling seven-day growth rate that has slowed but not yet reversed. Officials said the storm had set back vaccination logistics by interrupting cold-chain transport from Erbil and Kermanshah for nearly two days, and that mobile water-treatment units had been forced offline by clogged intake filters.

“You have a population that is simultaneously dehydrated, exposed to contaminated water, and now exposed to a respiratory irritant that is going to drive more people into clinics that are already saturated,” said Dr. Karim Soltani, the WHO epidemiologist who has been coordinating the agency’s field response from Ilam. “Triage is becoming a daily choice between cholera and pneumonia, and neither one waits.”

Climatologists at the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, who track shamal activity across the northern Gulf, said the May event was unusually intense for the season but consistent with a multi-year pattern of more frequent and longer-duration storms tied to soil moisture deficits in the Tigris-Euphrates basin. Dr. Faisal al-Otaibi, a senior researcher at the university, said preliminary modeling indicated the storm had drawn most of its dust load from desiccated agricultural land in eastern Syria and western Iraq that has lost roughly a third of its vegetative cover over the past decade.

“What we are seeing is not weather in isolation but a feedback loop,” Dr. al-Otaibi said. “Less rainfall, more bare soil, more dust uplift, more disruption to the people who would otherwise be farming that soil. The war did not cause this, but it has put a much larger and more vulnerable population directly in the path of it.”

UNICEF said it had begun distributing roughly 240,000 protective face masks across the Mehran-Khanaqin corridor on Thursday morning, with priority given to children, pregnant women and clinic staff. The agency said it had also moved forward the delivery of 18 portable oxygen concentrators and an emergency airlift of nebulizer supplies from its Dubai warehouse, originally scheduled for next week. UNHCR, in a parallel statement, said it had authorized the early release of more durable shelter materials — including sealed-floor tents and additional plastic sheeting — at the eight largest reception sites, replacing tarpaulin structures that had performed poorly during the storm.

Iranian authorities reported similar pressures on the western side of the border. The Ilam provincial health directorate said hospital occupancy in the provincial capital and in Mehran district had reached 96 percent on Wednesday, with elective procedures suspended for the third time in two weeks. State media said the Iranian Red Crescent had deployed an additional 340 personnel to the corridor and that the national emergency operations center in Tehran had requested emergency airlifts of saline, bronchodilators and pediatric ventilator circuits from its strategic reserves in Mashhad and Isfahan.

International donors are expected to review an updated humanitarian funding appeal at a coordination meeting in Amman next Tuesday, where WHO officials said the shamal-related caseload would be folded into a broader request that already covers cholera response, mine action and shelter. A senior European Commission official, speaking on condition of anonymity ahead of the meeting, said member states were now considering whether the corridor warranted a separate environmental health envelope distinct from the existing reconstruction track.

“The reconstruction conversation has been about cement and turbines,” the official said. “The story on the ground this week is about lungs. We will need to fund both, and faster than we planned.”

The Iraqi meteorological service said a second, weaker shamal pulse was possible over the weekend, and the WHO said field teams would maintain elevated surveillance for respiratory illness in the affected districts through at least the end of next week.