Authorities issued their highest warnings for fog and haze on Tuesday as smog enveloped major cities in northern China, warning the public that visibility could drop to less than 50 metres (164 feet). Northern province Hebei launched an anti-pollution emergency response, listing traffic safety controls for when necessary including suspending flight takeoffs and landings, temporarily closing highways and suspending ferries, China's meteorological bureau said in a notice. Authorities also warned road users to stop in safe parking areas when conditions required and asked people to stay indoors.