• Read Gas-powered leaf blowers are banned from neighborhoods — so why are they being used everywhere?

• Read What to know about 'mow and blow'

Blasting air at up to 185 mph, leaf blowers can whip up hazardous particles and contaminants from the ground at speeds greater than a Category 5 hurricane, sending them long distances.

Epidemiological studies have long recognized the harm these particles — including hydrocarbons from gasoline, animal droppings, spores, fungi, pollens, pesticides and herbicides, fertilizers, brake-lining dust and tire residue and heavy metals — cause to people's respiratory systems, according to Bay Area Air Quality Management District reports.

Exposure to particulate matter is rarely, if ever, cited as the cause of death in a coroner's report when someone dies of a heart attack or stroke or lung disease, a 2012 district study noted. "However, epidemiological studies indicate that exposure to particulate matter is an important contributing factor in hundreds, perhaps thousands, of deaths in the Bay Area each year."

The district has called particulate matter "the air pollutant that poses by far the greatest health risk to Bay Area residents."

The average adult inhales 450 cubic centimeters (roughly one pint) of air per breath, which includes 1 million to 10 million tiny particles with each breath.

"But that figure can spike to much higher levels in close proximity to high-volume roadways or other major outdoor emission sources," the district's "Bay Area 2010 Clean Air Plan" noted.

The contribution of leaf blowers to air pollution isn't to be underestimated. About 5 pounds of particulate matter per leaf blower per hour are swept into the air and take hours to settle, according to a widely cited leaf-blower pollution report by the Orange County, California grand jury in 1999.

An Air District program aimed at replacing up to 50,000 leaf blowers and 10,000 lawn mowers by 2020 would reduce the most dangerous small-particle emissions (sized 2.5 and 10 microns) by 0.12 tons (240 pounds) per day, according to the 2010 Clean Air Plan.

Fine particles measuring 2.5 microns and coarser material measuring 10 microns are more readily absorbed into the lungs. The smaller 2.5-micron particles are associated with hazardous organic compounds and heavy metals, according to the U.S. Environmental Protection Agency (E.P.A.). Particles measuring 10 microns are typically composed of smoke, dirt, dust, mold, spores and pollen.

Particulates in the 2.5-micron range can migrate many hundreds of miles and stay the air for days or weeks; 10-micron particles can travel up to 30 miles and stay aloft for hours, according the U.S. E.P.A.

Besides what they kick up off the ground, gas-powered leaf blowers themselves emit specific pollutants the State of California has identified as of concern: hydrocarbons from both burned and unburned fuel, which combine with other gases to form ozone; carbon monoxide; and toxic contaminants such as benzene, 1,3-butadiene, acetaldehyde and formaldehyde, according to a widely quoted 2000 California Environmental Protection Agency Air Resources Board report.

The Air Quality District in 2010 estimated there were approximately 258,000 two-stroke leaf blowers in the Bay Area, which generate significantly more air pollution than four-stroke engines.

Testing in 2011 by the vehicle reviewer Edmunds.com showed just how dirty leaf blowers remain, even 11 years after new emission standards for blowers went into effect.

Pitting leaf blowers against a Ford F-150 SVT Raptor crew cab, the leaf blowers were the big dogs when it came to spewing non-methane hydrocarbons, oxides of nitrogen and carbon monoxide — the three pollutants that the EPA and the California Air Resources Board find most concerning.

The two-stroke blower generated 23 times the carbon monoxide and nearly 300 times more non-methane hydrocarbons as the truck.

"To equate the hydrocarbon emissions of about a half-hour of yard work with this two-stroke leaf blower, you'd have to drive a Raptor for 3,887 miles, or the distance from northern Texas to Anchorage, Alaska," the article noted.

Officials from local lung-health organizations said the contribution of leaf blowers to pollution can't be ignored.

"It should be of great concern," said Lynn Smith, interim executive director of Breathe California of the Bay Area, also noting the huge discrepancy between leaf blower and car emissions.

Various arguments have been made by some environmental groups that blowers should be entirely banned in favor of a return to old-fashioned brooms and rakes.

A 1999 study by the University of California Riverside and San Joaquin Valley Unified Air Pollution Control District, the first of its kind, attempted to quantify the differences. Leaf blowers produced about 30 milligrams per square meter of 2.5-micron-sized particulates and 80 mg per square meter of 10-micron particles.

The results were similar for push brooms used on a concrete surface, probably because of the smoother surface, the researchers found. But using a push broom on asphalt produced no 2.5-micron particles and only 20 mg in the 10-micron range.

And raking on either surface produced no particulates in either range, the study found.

The California Landscape Contractors Association, however, disputes the allegations of the air pollution caused by leaf blowers, calling concerns over air emissions "spurious," according to a 1999 letter from its board of directors that was confirmed as current on July 20.

"Properly used leaf blowers do not raise inordinate amounts of dust. Rule 403 of the South Coast Air Quality Management District states that 'a person shall not cause or allow the emissions of fugitive dust from any active operation, open storage pile, or disturbed surface area such that the presence of such dust remains visible in the atmosphere beyond the property line of the emission source.' Blower users can and should follow this rule," the letter states.

In addition to arguing that emissions standards from the California Air Resources Board implemented in 2000 would significantly reduce emissions from handheld equipment, the association pointed to the intermittent use of blowers.

"Portable lawn and garden equipment contributes only 0.8 percent of all U.S. VOC (volatile organic compound) emissions, 0.6 percent of carbon monoxide emissions, and no nitrogen oxide emissions."

Debates over air pollution aside, there's also noise — perhaps the most evident pollution caused by leaf blowers. The City of Palo Alto requires leaf blowers to emit no more than 65 decibels, when measured from 50 feet away.

The Occupational Safety and Health Administration (OSHA) has determined that decibel levels above 85 cause permanent hearing loss. The World Health Organization recommends a general outdoor noise level of 55 decibels or less and 45 or less for sleeping restfully.

Excessive noise has been implicated in higher heart-attack rates, gastrointestinal disturbances, sleep problems, social discord and psychological problems, according to the U.S. E.P.A.

Ironically, metal rakes aren't much quieter, though the sound is less constant: The City of Palo Alto noted in a 2005 report that metal rakes used on concrete can generate 58-60 decibels at 50 feet.

When it comes to encouraging gardeners to forego their gas-powered machines, one air quality district in southern California has had significant success with its leaf blower exchange. The South Coast Air Quality Management District, which covers Orange County, urban Los Angeles, San Bernardino and Riverside, has held a leaf blower buy-back program since 2006 for professional gardeners. In that time, the district has put more than 12,000 reduced-noise and lower-emissions leaf blowers in the hands of professional gardeners.

The agency distributes about 1,500 new leaf blowers annually, said spokesman Sam Atwood.

"According to the E.P.A., a commercial blower emits 93 pounds per year of air pollutants. Multiplied out times 12,000, the units we have distributed have reduced 500 tons of pollutants since 2006," he said.

So far, the district has distributed cleaner blowers manufactured by the company Stihl. The company has supplied trainings at the exchanges. Operators learn to use the blower like a broom, rolling the debris from one area to another where it can be collected, rather than blasting it in a cloud of dust, he said.

The district helped support the development of backpack electric leaf blowers, which are just now becoming commercially available, he said. Atwood said the district hopes that it will get at least one proposal this year for a truly zero-emission, battery-powered leaf blower as part of its request for proposals.

"In demos, they seem to work very well, equal at least to a gas-powered blower. But it's a little premature to say how they will compare in the field to their gasoline counterparts," he said.

For its part, the Bay Area Air Quality Management District is running a program to fund the purchase of new, battery-powered, zero-emission electric lawn and garden equipment in exchange for gasoline-powered lawn and garden equipment. The program is currently only operating in Alameda and Contra Costa counties, however.