Bans on smoking in public parks are still fairly rare in the United States, despite more than half of states having indoor smoking restrictions, researchers say.

Just 355 counties out of 3,143 across the nation have smoke-free park policies - typically they're areas where the population is younger, more politically liberal and more well-off, the study team found. But rural and poorer communities should be encouraged to enact them too in the name of public health, they conclude.

"Air quality studies have demonstrated that smoking in outdoor areas such as building entrances or city streets is associated with measurable concentrations of secondhand smoke components, including nicotine and particulate matter," said the study's senior author Elizabeth G. Klein.

"Secondhand smoke exposure can cause cardiovascular damage in as little as 30 minutes, so even small amounts of exposure, indoor or outdoor, could be a health concern," said Klein, of the Division of Health Behavior and Health Promotion at the College of Public Health at Ohio State University in Columbus.

Banning smoking in public parks also helps to "denormalize" smoking, and hopefully leads to fewer young people picking up the habit, she said.

Twenty-eight U.S. states have enacted bans on smoking in all indoor public places, including bars and restaurants.

In three states, New Jersey, Delaware and California, more than half the counties have a policy designating city parks as smoke-free. New Jersey leads with more than 80 percent of counties having outdoor smoke-free policies.

Seven states, on the other hand, have no such policies: Alaska, Kentucky, North Dakota, South Dakota, Nevada, Virginia and the District of Columbia.

Areas with lower incomes and education levels were least likely to have smoke-free policies, Klein's team reports in Nicotine & Tobacco Research.

"For some outdoor environments, like a patio, a bus stop, or a crowded beach, protection from exposure to tobacco smoke pollution is a solid rationale," Ryan David Kennedy said.

"For other environments - like parks - where exposure might be more avoidable, the reasons behind bans have had more to do with 'social protection,' the idea that we don't want young people to think smoking is a socially acceptable behavior," he said.

Kennedy is a tobacco control researcher at Johns Hopkins Bloomberg School of Public Health in Baltimore and was not involved in the new study.

The cities on the road to having outdoor smoking bans now were also among the first to have indoor bans, and it makes sense that policy would evolve that way, he said.

"Communities that face deep economic and health disparities may not have smoke-free outdoor spaces on the top of their list, particularly if indoor restrictions are not in place," Kennedy said.

"Given that parks provide the means for more physical activity and a connection with nature, it is important to make them smoke-free," said Lambros Lazuras, a health and wellbeing researcher at the South-East European Research Centre in Thessaloniki, Greece.

"This way, people from all age groups who frequent parks would be more likely to associate these places with healthy images, and in the longer term this can pay off as an important community-wide strategy for healthy and active lifestyles," he said.

In previous research in Minnesota, Klein found that 70 percent of residents favored smoke-free policies in parks, since they would decrease litter, reduce secondhand smoke and keep kids out of harm's way.

She also surveyed park directors: more than 90 percent of those without an outdoor smoking policy worried that enforcing one would be tough, but only 26 percent of directors at parks with a smoke-free policy reported having problems.

"We believe these results suggest a high degree of support for smoke-free parks among residents, and fears of policy difficulties among park and recreation directors who work in parks without a tobacco-free policy were much greater than actual problems experienced in Minnesota tobacco-free park areas," Klein said.

"Smoking continues to be the leading cause of death and disease in the United States - and smoke-free policies are associated with 'denormalizing' smoking, as well as supporting people who are trying to quit," Kennedy said.