Monsters aren’t just hiding under the bed or in closets; today they’re taking to the streets of San Francisco in support of a statewide ban on plastic bags.

Donning 500 plastic bags each, 100 people dressed as “bag monsters” will march from Ghirardelli Square to Fisherman’s Wharf and back.

A number of organizations will participate, including Save the Bay and the Plastic Pollution Coalition.

State lawmakers are weighing Assembly Bill 1998, which would ban single-use plastic bags. The Legislature is set to vote on the bill this month.

The designer behind the non-biodegradable costumes, Andy Keller, has been touring the country with his creations and plans to visit more than a dozen cities that are considering implementing bag ban legislation.

Clad as the bag monster and riding across the continent on a three-wheeled Spyder motorcycle, Keller’s next stop on a 15-city cross-country tour is Eugene, Ore.

Keller will tour cities including Portland, Seattle, Chicago and New York. In each of those cities, Keller plans to drape the costumes over a 9-foot-tall frame, creating a giant bag monster.

In 2007, San Francisco became the first city in the U.S. to introduce a ban on non-biodegradable plastic bags, requiring large pharmacies and grocery stores to replace them with reusable or recyclable bags.

A typical individual uses an average of 500 bags annually, according to Keller.

Last week, San Francisco Supervisor Ross Mirkarimi introduced legislation to expand that ordinance to include smaller markets, restaurants, liquor stores, convenience stores and “big box” retailers.

The ordinance would become the strictest in the nation, if passed by the board.

The state ban would go into effect Jan. 1, 2012, at certain businesses like supermarkets and large retailers, with an extension to convenience food stores and smaller stores a year and a half later.

A 1 p.m. news conference at Ghirardelli Square is scheduled to follow the noon march.

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