May 1 marks the beginning of the 20th "Spare the Air" ozone season, which is typically an issue for the Valley from now until the end of October.
Weather patterns during this time period promote ozone pollution. The sun's ultraviolet rays reach their peak intensity, reacting with volatile organic compounds (VOC) and oxides of nitrogen (NOx) to form ground-level ozone. Sources of VOC and NOx are cars, trucks, buses and construction/agricultural equipment. The late afternoon is the time of day when air quality tends to be the worst.
The Central Valley is a breeding ground for poor air in the summer. Typical summer pattern of high pressure allows air to stagnate for several days or weeks leading to decreasing air quality. The middle altitude becomes warmer than the ground, producing an inversion. This warmer air acts like a blanket over the area resulting in minimal vertical mixing. Light low level winds can't adequately disperse the pollutants so they stay in the valley.
The Sacramento and San Joaquin Air Quality Management districts monitor this air quality. They first measure pollutants, usually ozone, in parts per billion. That number is then translated into an air quality number indicating the different levels of tolerance.
High levels of ground-level ozone can lead to health problems especially in the elderly, children and people who have compromised respiratory systems. Wheezing, coughing, fatigue, headache and nausea can all be side effects from poor air quality.
Air quality typically improves by November because cooler weather systems move through Northern California. The upper air movement and precipitation keep the pollutants from stagnating.