Ozone Season 'Halftime' Update: Exceedance Days Increased Over 2020

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Kevin J MacDonald, Public Information Administrator
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Tuesday, July 27, 2021

Ozone Season 'Halftime' Update: Exceedance Days Increased Over 2020
Ground-level ozone in Clark County was higher during the first half of “ozone season” this year, compared to previous years, according to the Department of Environment and Sustainability (DES). From April to June, the department’s Division of Air Quality recorded 11 days when ozone levels exceeded the U.S. Environmental Protection Agency’s ozone standard. While exceedance days increased, the average monthly air quality index figures are consistent with ozone concentration levels in previous years. Air Quality officials cite a combination of factors for the ozone exceedances.

“Wildfire smoke, locally-produced ozone and transport of pollutants have all been factors in our exceedance days, so far,” said Paul Fransioli, senior air quality specialist. “It’s been a mix of at least two of those three elements in our exceedances this year.”
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Vehicle emissions are the leading cause of air pollution in Nevada, but record-setting temperatures are also contributing to ozone creation. In addition to this summer’s extreme heat, three of the previous five years were the hottest on record for Las Vegas. Wildfires throughout the southwest have also been prevalent during four of the last five summers, including 2021.

“Transport of pollutants from elsewhere, wildfire smoke and increased heat due to climate change will require a robust, regional response to help curb air pollution,” said DES Director Marci Henson. “Our permitting and compliance enforcement actions keep our local business community operating within all federal, health-based regulations, but what happens in other parts of the country also impacts Clark County’s air quality.”

All data gathered by the Clark County Department of Environment and Sustainability's (DES) Division of Air Quality. It has not been officially verified and is subject to change. Though this is the most current data, it is not official until it has been certified by our technical staff. Data is collected from DES ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.

DES designates April 1 to Sept. 30 each year as its “ozone season,” when concentrations of ground-level ozone are higher than during the remaining months of the year. Ground-level ozone is toxic to breathe. According the EPA’s health-based standards, an exceedance occurs when ozone particles are greater than 70 parts per billion over a rolling eight hours in a day.

How Does Ozone Form?
Ground-level ozone is formed when volatile organic compounds (VOCs) mix with oxides of nitrogen (NOx) in ultraviolet rays and heat. VOCs include fumes from fuel, paints and chemicals, and even some plants and vegetation. 

Examples of NOx are vehicle exhaust, factory emissions and wildfire smoke. Stagnant weather conditions and the topographic structure of the Las Vegas Valley help trap these pollutants, causing levels to rise. Exposure to ozone can irritate your respiratory system and cause coughing, a sore throat, chest pain and shortness of breath even in healthy people, according to the EPA.

Air Quality Index
The Air Quality Index (AQI) is a measurement tool to gauge air pollution. It runs from 0 – 500 and is used to measure six pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulfur dioxide. The higher the AQI value, the greater level of air pollution and greater the health concern. 

About the Department of Environment and Sustainability
The Department of Environment and Sustainability is the air pollution control agency, regional Endangered Species Act compliance program, and sustainability office for all of Clark County, Nevada. Established as the Department of Air Quality by the Clark County Commission in 2001, it was renamed in 2020 and is comprised of three divisions: Air Quality, Desert Conservation Program and Office of Sustainability. Through these three divisions, DES is ensuring the air we share meets healthful, regulatory standards, administering the County's Multiple Species Habitat Conservation Plan and addressing climate change.


Clark County is a dynamic and innovative organization dedicated to providing top-quality service with integrity, respect and accountability.  With jurisdiction over the world-famous Las Vegas Strip and covering an area the size of New Jersey, Clark is the nation’s 11th-largest county and provides extensive regional services to 2.4 million citizens and 45.6 million visitors a year (2019). Included are the nation’s 7th-busiest airport, air quality compliance, social services and the state’s largest public hospital, University Medical Center. The County also provides municipal services that are traditionally provided by cities to 1.1 million residents in the unincorporated area. Those include fire protection, roads and other public works, parks and recreation, and planning and development.