The Qalb Loze massacre was a reported massacre of Syrian Druze on 10 June 2015 in the village of Qalb Loze in Syria's northwestern Idlib Governorate.
 The village was under the control of a coalition of Islamist rebels, when a Tunisian commander of one group in the coalition, the al-Nusra Front, tried to confiscate the house of one of the villagers who they accused of working for the Syrian government.
 The villagers protested and there were two different accounts of what followed.
 According to anti-government activists, the Nusra fighters opened fire on the protesting villagers, after the Tunisian commander accused them of blasphemy.
 In contrast, Nusra member Mohammad Feezo claimed it was the villagers who opened fire first.
 In the end, the al-Nusra Front killed 20 people, including the elderly and a child.
 The Druze were reported to have killed three members of al-Nusra Front, including the Tunisian.
 Two days later, a report put the number of Druze killed at 24.
Dhokha is a Hindi film that stars Muzammil Ibrahim and Tulip Joshi in the lead roles.
 It is directed by Pooja Bhatt, produced by Mukesh Bhatt and music by M. M. Keeravani.
 The film revolves around a moderate Indian Muslim cop, who wakes up and to his horror, discovers that his wife, who had recently been killed in a bomb blast, is accused of being a suicide bomber and had killed 20 people in the club.
The 1948 Donora smog was a historic air inversion that resulted in a wall of smog that killed 20 people and sickened 7,000 more in Donora, Pennsylvania, a mill town on the Monongahela River, 24 mi southeast of Pittsburgh.
 The event is commemorated by the Donora Smog Museum.
The 2013 Atimonan road crash was a road accident involving three buses and five other vehicles which occurred on a zigzag diversion in Maharlika Highway in Barangay Santa Catalina, Atimonan, Quezon province.
 The road accident killed 20 people and another 54 were injured.
The 1881 Minnesota tornado outbreak was a deadly tornado outbreak that struck southern Minnesota on July 15–16, 1881.
 At least six tornadoes touched down between 2:00 pm – 6:00 pm CST, killing 24 people and injuring at least 123.
 The deadliest tornado of the outbreak, an F4 that killed 20 people in and near New Ulm, was likely a tornado family that may have caused F5 damage to rural farmsteads.
 Six people died in New Ulm, where people from nearby settlements had congregated to avoid Native American attacks.
In the 2009 Guinea mine collapse in May 2009, a cave-in at a gold mine in Siguiri, Guinea, killed 20 people, injured five and left ten missing.
The Banjarsari massacre occurred on 15 April 1987, when a 42-year-old farmer named Wirjo killed 20 people and wounded 12 others in the village of Banjarsari in Banyuwangi Regency, Indonesia.
The Chicago Fire of 1874 was a conflagration in Chicago, Illinois, that took place on July 14, 1874.
 Reports of the extent of the damage vary somewhat, but sources generally agree that the fire burned forty-seven acres just south of the Loop, destroyed 812 structures and killed 20 people.
 The affected neighborhood had been home to Chicago's community of Jewish immigrants from Russia and Poland, as well as to a significant population of middle-class African-American families; both ethnic groups were displaced in the aftermath of the fire to other neighborhoods on the city's West and South Sides.
Donora Smog Museum features a collection of archival materials documenting the Donora Smog of 1948, an air inversion of smog containing fluorine that killed 20 people in Donora, Pennsylvania, United States, a mill town 20 miles south of Pittsburgh on the Monongahela River.
Mary Ochsenhirt Amdur (February 18, 1921 – February 16, 1998) was an American toxicologist and public health researcher who worked primarily on pollution.
 She was charged with studying the effects of the 1948 Donora smog, so she specifically looked into the effects of inhaling Sulfuric acid by experimenting on guinea pigs.
 Her findings on the respiratory effects related to sulfuric acid led to her being threatened, to her funding being pulled, and to her losing her job at the Harvard School of Public Health in 1953.
 Undeterred by the setback, she carried on her research in a different role at Harvard, and subsequently at MIT and New York University.
 Despite the early controversy related to her work, it was used in the creation of standards in air pollution, and towards the end of her life she received numerous awards and accolades.

Question: Mary Ochsenhirt Amdur studied the effects of which historic air inversion that killed 20 people?
Answer: 1948 Donora smog