Air pollution regulators are warning thousands of Bay Area employers they could be fined for failing to comply with a rule requiring them to offer a commuter benefit to employees who get to work via van pool, bus, train or bike.

Under the 2014 rule made permanent last year, employers with 50 or more full-time workers must offer them a benefit encouraging commute methods that reduce gridlock and air pollution.

The Bay Area Air Quality Management District has estimated that about 8,000 employers are covered by the rule, but only about 4,200 have registered with the air district and demonstrated they offered a benefit, officials said Tuesday.

“Employees are more likely to change their commuting behavior if it is encouraged and promoted by their employer,” said Jack Broadbent, executive officer of the nine-county air district.

The benefit can save employees several hundred dollars a year, as well as lower payroll taxes for employers, according to the air district and the Metropolitan Transportation Commission.

Continued noncompliance could result in companies being cited and fined, said Tom Flannigan, an air district spokesman.

“Our first option will be working with companies to get them to comply,” he said, “but companies at some point could be cited for violations just like businesses that pollute.”

Companies can register at 511.org, and find more out more information about it at http://511.org/employers/commuter/news.

Under four options, employers can allow workers to exclude up to $255 a month of their transit or van pool costs from taxable income, or provide a subsidy of up to $75 a month to cover employees’ public transit or and van pool costs.

Employers also can offer workers a free or subsidized bus or shuttle service such as buses offered to Google workers.

Employers also can come up with their own option equally as effective as the first three options.

The air district commuter benefit rule was modeled after commuter benefit ordinances established in San Francisco, Berkeley and Richmond.

The air district started its commuter benefit rule as a three-year pilot program, but California lawmakers in 2016 passed a law allowing the rule to become permanent.