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Home Air Criteria Pollutants

Carbon Monoxide 1 Hour Concentration

Status and Trend

Interpretation and Commentary

Considerably Better Than Target : Rapid Improvement : High Confidence
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Status: Considerably Better Than Target
Trend: Rapid Improvement
Confidence: High

  • Relevance - Carbon monoxide (CO) in high concentrations affects human health by reducing the supply of oxygen to the tissues of the body. Health effects can include headaches, nausea, reduced mental alertness and even death at very high concentrations. Federal and state standards, with varying time averaging periods, have been adopted to protect the public from this harmful pollutant. This indicator addresses the highest and second highest monitored CO concentrations averaged over every 1 hour for the calendar year.
  • Adopted Standards  - California standard: Maintain 1-hour concentrations of carbon monoxide at or below 20 ppm, Nevada and federal standard: Maintain 1-hour concentrations of carbon monoxide below 35 ppm, TRPA has not adopted a 1-hour carbon monoxide Threshold Standard.
  • Indicators - Highest and second highest 1-hour concentration in parts per million (ppm) of carbon monoxide measured within a calendar year. Only the highest 1-hour concentration is used to determine compliance with adopted Threshold Standards. The 2nd highest is provided to demonstrate the magnitude of difference between the highest and 2nd highest recorded 1- hour concentration in the Region.
  • Condition Status – The Region has been in compliance with the strictest adopted Threshold Standard since 1983, including 2010. The 2010 first high and second high 1 hr. carbon monoxide (CO) concentration values were 4.0 ppm and 3.2 ppm respectively1. The first high value is 20% of the most stringent (CA) standard, of not to equal, or exceed 20 ppm. The second high value is slightly more than 9% of the federal and NV standards of 35 ppm. A status of “considerably better than target” was designated.
  • Trend – Trends were calculated using the Thiel regression method which is academically well accepted for air quality analysis.
    • Long Term Trends - The long-term trend indicates a reduction in highest recorded 1-hour average CO concentrations at the Horizon site between 1983 and 1998, and the Harvey’s site between 2000 and 2010. On average, 1-hour CO concentrations have decreased by -0.5 ppm/year at the Horizon site and -0.9 ppm/year at the Harvey’s site. These rates are -2.5% per year and -4.5% per year of the most stringent (CA) standard of 20 ppm, respectively. Consequently, the long-term trend determination is “rapid improvement”.
    • 5 Year Trend - The trend over the most recent 5 years (2006-2010) is not consistent with the long term trend and instead indicates no change in first high CO concentrations.  This indicator has been in attainment with the strictest 1-hour average CO concentration standard since monitoring efforts began in 1983.

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  • Confidence
    • Condition Status - There is high confidence in the condition status because the data is collected using federal reference methods2, are subject to quality assurance requirements, and were collected consistently between 1983 and 2010 with the exception of moving the monitoring site approximately ¼ mile in 1999. Only one monitoring site is used to determine indicator status. However, the monitoring site is located at the south shore casino core. This site represents the greatest volume of vehicle traffic in the Region, and consequently the measurements are thought to represent the highest CO emissions.
    • Long Term Trends - The confidence in the trend for the first high values at the Horizon site is high for the long range analysis of 16 data points with a confidence level of 100%, a S-value of -89 and a P value < 0.01. The confidence in the trend for the first high values at the Harvey’s site is also high for 11 data points with a confidence level of 98%, a S-value of -28 and a P value of 0.02.
    •  5 Year Trend - The confidence in the trend for first high over the most recent 5 years (2006-2010) is low with a confidence level of 41%, S value of 0 and a P value of 0.59. Low confidence is likely the result of few data points (n=5) and inter-annual variation in 1-hour concentrations.
    •  Overall Confidence - The overall confidence is high because there is high confidence in the condition status and long-term
      trends. Although there is low confidence in the short-term trend, data for the last five years indicate 1-hour CO concentration have remained well below the strictest standard.
  • Human and Environmental Drivers - Carbon monoxide is emitted from incomplete fuel combustion by sources such as cars, trucks, boats, construction equipment, fireplaces, woodstoves, furnaces and wildfire. The ambient concentration of CO is highly dependent on meteorological conditions such as temperature, wind speed and mixing conditions.
  • Monitoring Approach – Between 1983 and 1998, carbon monoxide was monitored at the Horizon Hotel in Stateline, NV. In 1999,
    the monitoring site was relocated to Harvey’s Resort parking garage in Stateline, NV. The site is located to monitor the highest CO
    concentrations in the Lake Tahoe Basin because historically this area received the high vehicle traffic volume (NDOT and Caltrans,
    and is intended to be representative of both the CA and NV sides of the south shore resort district. Data are collected, analyzed and
    reported by the Nevada Division of Environmental Protection (NDEP 2011)3.
  • Monitoring Partners – Nevada Division of Environmental Protection, U.S. Environmental Protection Agency, and Tahoe Regional Planning Agency


  • Monitoring Plan
  • Conceptual Model


Carbon Monoxide Monitoring Locations in the Lake Tahoe Basin

Carbon Monoxide monitoring locations

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Trend Charts

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Additional Info


  1. U.S. Environmental Protection Agency, AirData website, April 25, 2011: http://www.epa.gov/aqspubl1/.
  2. EPA methods for air quality monitoring: http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf
  3. Nevada Air Quality Trend Report, 1998-2009. Nevada Division of Environmental Protection, Bureau of Air Quality Planning, January 2011. http://ndep.nv.gov/baqp/monitoring/docs/trend.pdf

Additional Information

  1. Carbon Monoxide 8 hr avg. Indicator
  2. CARB fact sheet on health effects: http://www.arb.ca.gov/html/fslist.htm#Health
  3. Conceptual Model: A conceptual model for Air Quality CO does not exist as of June 2011 and is anticipated to be developed in 2011.
  4. Monitoring Plan: A monitoring plan for Air Quality CO does not exist as of June 2011 and is anticipated to be developed in 2012
Last Updated on Monday, 05 November 2012 10:30