An unseasonal sandstorm has swept across the Middle East covering Beirut and Damascus and causing the deaths of at least five people.
Hundreds more were admitted to hospital with breathing difficulties, according to officials, as cities were blanketed in yellow dust.
Reduced visibility prompted the Syrian government to call off airstrikes against rebel fighters in a central province, local media reported, and threatened a planned protest by Lebanese activists over the government's inability to deal with the country's rubbish crisis.
The storm also hit Jordan, Israel and Egypt. In Jordan, schools shut down or cut their days short.
The sandstorm reached Beirut on Tuesday, a day after it engulfed eastern Lebanon's Bekaa Valley. People, especially those with health issues, were advised to stay indoors while many of those who ventured onto the streets donned surgical masks.
The Lebanese Health Ministry said 750 people suffered breathing problems across the country, and that two women died because of the sandstorm.
Two boats set adrift were rescued by coastguard, the National News Agency said. Airport officials reported some flight delays.
Lebanese authorities warned residents against burning rubbish that has piled up on Beirut streets this summer, sparking a political crisis and daily protests.
Lucien Bourjeili, one of the protest organisers, said the bad weather may prevent some people from taking to the streets in a major protest planned for Wednesday.
In the Syrian capital, Damascus, the head of a major hospital, Adeeb Mahmoud, said more than 1,200 people, including 100 children, had been treated for breathing problems.
The Syrian pro-government Al-Watan newspaper said the weather forced a halt in government airstrikes against rebel fighters north of the central province of Hama.
The British-based Syrian Observatory for Human Rights, said hospitals in the town of al-Mayadeen in the northern province of Deir el-Zour ran out of oxygen cylinders and were unable to take in more patients.