LAGOS, Nigeria - A build up of pollution from factories and old cars was the cause of a wave of smog that enveloped much of Nigeria's largest city, a government spokesman said Thursday.

Newspapers described the unprecedented level of smog as "mystery smoke" and "mystery fumes."

But government scientists who analyzed the fumes determined they resulted from the build up of emissions from factories and old cars, Lagos state spokesman Kehinde Bamgbetan said.

"The climate became a little bit colder, so it enabled the polluted air to develop into a smog," Bamgbetan said.

No one died from the smog, he said.

Lagos state government has told industries they will be shut down if they do not submit environmental audit reports by the end of the year, he said.