Question: In what month of 1956 was the event that caused The Clean Air Act as a response?

Context articles: National Emissions Standards Act
Clean Air Act 1956
State Implementation Plan
Air Pollution Control Act
New Brunswick environmental legislation
Clear Skies Act of 2003
New Source Performance Standard
Oregon Department of Environmental Quality
Great Smog of London
Volkswagen emissions scandal

The National Emissions Standards Act, officially known as the Motor Vehicle Air Pollution Control Act (Pub.L.
 89–272 ), is a 1965 amendment to the U.S. Clean Air Act of 1963.
 The amendment set the first federal vehicle emissions standards, beginning with the 1968 models.
 These standards were reductions from the 1963 emissions: 72% reduction for hydrocarbons, 56% reduction for carbon monoxide, and 100% reduction for crankcase hydrocarbons.
 The impact the regulatory standards will have on air quality in the future, as well as the potential characteristics of the vehicle fleet can be analyzed with the use of roadway air dispersion models.
The Clean Air Act 1956 was an Act of the Parliament of the United Kingdom passed in response to London's Great Smog of 1952.
 It was in effect until 1964, and sponsored by the Ministry of Housing and Local Government in England and the Department of Health for Scotland.
A State Implementation Plan (SIP) is a United States state plan for complying with the federal Clean Air Act, administered by the Environmental Protection Agency (EPA).
 The SIP, developed by a state agency and approved by EPA, consists of narrative, rules, technical documentation, and agreements that an individual state will use to clean up polluted areas.
The Air Pollution Control Act of 1955 (Pub.L.
 84–159 , ch.
 360, 69 Stat.
 322 ) was the first Clean Air Act (United States) enacted by Congress to address the national environmental problem of air pollution on July 14, 1955.
 This was "an act to provide research and technical assistance relating to air pollution control".
 The act "left states principally in charge of prevention and control of air pollution at the source".
 The act declared that air pollution was a danger to public health and welfare, but preserved the "primary responsibilities and rights of the states and local government in controlling air pollution".
The province of New Brunswick has created and implemented various Acts, such as the Clean Environment Act, Clean Air Act, and the Clean Water Act, throughout history to ensure that the environment is considered and protected throughout various project.
 These acts describe the process that will be undertaken to assess the impact of projects, and the steps to be taken when certain situations arise.
The Clear Skies Act of 2003 was a proposed federal law of the United States.
 The official title as introduced is "a bill to amend the Clean Air Act to reduce air pollution through expansion of cap-and-trade programs, to provide an alternative regulatory classification for units subject to the cap and trade program, and for other purposes."
New Source Performance Standards (NSPS) are pollution control standards issued by the United States Environmental Protection Agency (EPA).
 The term is used in the Clean Air Act Extension of 1970 (CAA) to refer to air pollution emission standards, and in the Clean Water Act (CWA) referring to standards for water pollution discharges of industrial wastewater to surface waters.
The Oregon Department of Environmental Quality (DEQ) is the chief regulatory agency of the government of the U.S. state of Oregon responsible for protecting and enhancing the state's natural resources and managing sanitary and toxic waste disposal.
 The agency employs approximately 700 scientists, engineers, technicians, administrators, and environmental specialists.
 It has headquarters in Portland, regional administrative offices in Bend, Eugene, and Portland; and field offices in Coos Bay, Grants Pass, Hermiston, Medford, Pendleton, Roseburg, Salem, and The Dalles.
 The Laboratory and Environmental Assessment Division operates an environmental laboratory in Hillsboro.
 The agency's director has the authority to impose fines for violations of the state's anti-pollution laws.
 In addition to its state mandate, the United States Environmental Protection Agency (EPA) has delegated authority to DEQ to administer federal environmental programs including the federal Clean Air Act, Clean Water Act, and Resource Conservation and Recovery Act within the state.
The Great Smog of London, or Great Smog of 1952 sometimes called the Big Smoke, was a severe air-pollution event that affected the British capital of London in December 1952.
 A period of cold weather, combined with an anticyclone and windless conditions, collected airborne pollutants – mostly arising from the use of coal – to form a thick layer of smog over the city.
 It lasted from Friday, 5 December to Tuesday, 9 December 1952 and then dispersed quickly when the weather changed.
The Volkswagen emissions scandal (also called "emissionsgate" or "dieselgate") started on 18 September 2015, when the United States Environmental Protection Agency (EPA) issued a notice of violation of the Clean Air Act to German automaker Volkswagen Group.
 The agency had found that Volkswagen had intentionally programmed turbocharged direct injection (TDI) diesel engines to activate some emissions controls only during laboratory emissions testing.
 The programming caused the vehicles' NO output to meet US standards during regulatory testing but emit up to 40 times more NO in real-world driving.
 Volkswagen deployed this programming in about eleven million cars worldwide, and 500,000 in the United States, during model years 2009 through 2015.