Course Outline and Key Points - Ozone
On this page:
- What is Ozone?
- Health Effects of Ozone in the General Population
- Health Effects of Ozone in Patients with Asthma and Other Chronic Respiratory Disease
- Patient Exposure and the Air Quality Index
What is Ozone?
What is ozone and where is it in the atmosphere?
- Ozone is a highly reactive gaseous molecule consisting of three oxygen atoms
- Ozone occurs naturally in the stratosphere (upper atmosphere)
- In the troposphere (lower atmosphere), ozone is formed primarily from photochemical reactions of man-made air pollutants
- Reaction to form ozone typically depend on the presence of heat and sunlight resulting in higher ambient ozone concentrations in the summer
- There are specific circumstances where high ozone concentrations can occur in cold weather as has been observed in a few high elevation areas in the Western U.S.
- Tropospheric ozone is also called "ambient" or "ground-level" ozone
Are high ambient ozone concentrations found only in urbanized areas?
- Ozone and its air pollutant precursors can be transported long distances, so high ground- level ozone concentrations may occur in rural as well as urban areas
- Ozone concentrations usually peak during afternoon hours when sunlight is strongest, but ozone peaks may also occur late in the day after wind has carried ozone and its precursors miles from their sources
How does atmospheric ozone affect human health?
- Ozone absorbs ultraviolet (UV) light, reducing exposure to harmful radiation that causes skin cancer and cataracts
- Ground-level ozone that is inhaled damages the respiratory tract, leading to a number of adverse health effects that are addressed in the following material
Health Effects of Ozone in the General Population
- Breathing ground-level ozone can result in health effects that are observed in broad segments of the population, including respiratory symptoms, reduced lung function, and airway inflammation, as well as more serious effects such as increased hospital admissions and increased daily mortality.
- Respiratory symptoms can include coughing; throat irritation; pain, burning, or discomfort in the chest when taking a deep breath; chest tightness, wheezing, or shortness of breath
How are people exposed to ozone?
- Ozone exposure occurs when people breathe ambient air containing ozone
- The rate of exposure for a given individual is related to the concentration of ozone in the surrounding air and the amount of air the individual is breathing per minute
- Ozone concentrations are higher outdoors than indoors
- Increased physical activity increases a person's rate of ozone exposure
How does ozone react in the respiratory tract?
- The upper respiratory tract is not as effective in scrubbing ozone from inhaled air as it is for more water soluble pollutants
- Consequently, the majority of inhaled ozone reaches the lower respiratory tract
- Ozone reacts with chemical constituents of airway lining fluid throughout the respiratory tract, producing very reactive chemical intermediates
What are ozone's acute physiological and symptom effects?
- Ozone or its reactive products stimulate airway neural receptors, resulting in respiratory symptoms and decreases in lung function
- Effects are reversible, with improvement and recovery to baseline varying from a few hours to 48 hours after an elevated ozone exposure
What effects does ozone have at the cellular level?
- Ozone or its products damage airway epithelial cells which results in inflammation, an increase in nonspecific airway reactivity, and an increase in epithelial permeability
- Over a period of days to weeks, inflammation subsides and cell damage is repaired
How does response vary among individuals?
- A wide range of responsiveness to ozone exists among otherwise healthy people
- With regard to lung function, young adults (teens to thirties) and those with a high body mass index are more responsive than older adults (fifties to eighties) and those with a low BMI
What are the effects of ozone on mortality?
- Ozone is associated with increased mortality
- The absolute effect of ozone on mortality is considerably higher in older adults
- The ozone-mortality relationship is most prominent during the warm season
What are the other potential effects of short-term ozone exposure?
- High daily ambient ozone concentrations are associated with increased hospital admissions and emergency room visits as well as school absence.
At what exposure levels are effects observed?
- The concentration of ozone at which effects are first observed depends on the level of sensitivity of the individual as well as the dose delivered to the respiratory tract
- Dose = ambient concentration x level of exertion (minute ventilation) x duration of exposure
- An average outdoor laborer doing intermittent work might experience small to moderate lung function and symptom effects as well as lung injury and inflammation following an 8-hour exposure to 60 to 70 ppb ozone
- More sensitive individuals will experience such effects at lower concentrations while less sensitive individuals will experience these effects only at higher concentrations
- Children are less likely to recognize and report respiratory symptoms
- Field studies show effects at much lower levels of exposure than chamber studies
- Asthma attacks seem to increase when 8-hour average concentrations rise above 60 to 80 ppb
- Limited exposure-response modeling suggests that if a population threshold for ozone effects exists, it is likely near the lower limit of ambient ozone concentrations in the United States.
What are the effects of recurrent or long-term exposure to ozone?
- Airway injury continues to occur with ongoing, recurrent exposure
- Some evidence suggests that long-term exposure may be related to the induction of new cases of asthma and that recurrent exposure of young children may result in abnormal development of their respiratory systems
Health Effects of Ozone in Patients with Asthma and Other Chronic Respiratory Disease
- People with pre-existing respiratory diseases such as asthma are especially susceptible to ozone exposure
- Because the prevalence of asthma in children is particularly high and because children are generally at higher risk due to time spent being active outdoors, they may be disproportionately affected by ozone exposure
- On days when ozone levels are high, people with asthma tend to experience lung function decrements, and increases in respiratory symptoms, medication use, frequency of asthma attacks and use of health care services
- Asthma is a chronic inflammatory disease of the airways that causes recurrent episodes of wheezing, breathlessness, chest tightness, and cough
- The majority of asthma is associated with allergic responses to common airborne allergens such as household dust mites, pollens, animal dander, and molds
- Induction of asthma refers both to the acquisition of immunologic sensitivity to allergens and the progression to a clinically detectable disease
- About 23 million people, including almost 7 million children, have asthma
- An average of 1 out of every 10 school-aged children has asthma
- 2011 data from the Centers for Disease Control indicate an asthma prevalence rate of 8.4% in the United States
- The annual economic cost of asthma is about $20 billion. Direct costs make up about $15 billion of that total, and indirect costs such as lost productivity add another $5 billion
- There is a growing interest in the association between the environment and asthma
How does ozone affect people with asthma?
- People with asthma are likely affected by ozone in two ways: (1) they might be more sensitive to ozone than other people and experience lung function changes and respiratory symptoms at lower concentrations or with greater magnitude; and (2) the injury, inflammation, and increased airway reactivity induced by ozone exposure may worsen a person's underlying asthma, increasing the probability of an asthma attack. The second mechanism is by far the greater concern.
- Ozone exposure is associated with exacerbation of existing asthma and other chronic lung diseases
- Respiratory symptoms and bronchodilator use are increased, and lung function is decreased, on days when ozone concentrations are high
- Numbers of emergency room visits and hospital admissions for asthma are increased on days when ozone concentrations are high
COPD and other chronic respiratory disease
- COPD is the one other respiratory disease for which a relationship has been observed between ozone and hospital admissions
- In controlled laboratory exposures, patients with COPD have been observed to experience changes in oxygen saturation which may contribute to an exacerbation of COPD
Patient Exposure and the Air Quality Index
Should I recommend that my patients reduce their exposure to ozone?
- Evidence indicates that reducing a person's exposure to ozone will reduce the associated health effects
- People with asthma or other respiratory diseases will benefit most from exposure reductions
- Evidence also indicates that reducing ozone exposure reduces the public health impacts of associated asthma morbidity
- EPA recommends that health care providers counsel patients to reduce their ozone exposure when air quality is bad
How can my patients reduce exposure to ambient ozone and the consequent health effects?
- People can reduce their exposure by reducing: (1) the time they spend outdoors; (2) the level or duration of outdoor activity when ozone levels are high; or (3) a combination of (1) and (2)
- Patients can find out when to reduce their exposure to ozone by paying attention to air quality reports and forecasts that notify the public when ozone levels become unhealthy
What is the Air Quality Index (AQI)?
- The Air Quality Index, or AQI, is a nationally uniform index for reporting daily air quality in an area
- The AQI is a normalized scale that ranges from 0 to 500. The higher the AQI value, the greater the level of pollution and the greater the health concern
- When AQI values are above 100, air quality is considered to be unhealthy, first for sensitive groups (101-150), and then for the entire population (>150)
- When the air is forecast to be unhealthy, people should limit their outdoor activities as the AQI for ozone indicates, e.g., at 101-150, sensitive groups such as active children and adults, and people with respiratory disease, should limit prolonged or heavy exertion outdoors
What can I advise my patients to do when air quality is forecast to be unhealthy?
- Physicians should counsel their patients to use AQI forecasts to plan outdoor activities
- People can reduce their exposure to ozone by rescheduling outdoor activities to times of day when ozone levels are forecast to be lower (e.g., early morning), taking it easier when ozone levels are high, and spending less time engaged in vigorous outdoor activities when ozone levels are high
- People should pay attention to symptoms such as coughing, chest pain with a deep breath, chest tightness, and shortness of breath. These symptoms indicate that exposure reduction is warranted
How effective are air quality alerts in reducing adverse effects in the real world?
- The United States public is generally aware of the AQI, and people say they have taken steps to limit exposure when the air is forecast to be bad
- There is evidence that heeding air quality advisories reduces asthma morbidity
Where can the daily AQI be found?
AQI forecasts can be found in the following locations:
- With the weather forecasts in local newspapers, TV, and radio
- Through the national news media, including USAToday, The Weather Channel, and CNN
- On the EPA Web site www.airnow.gov
How can my patients get the daily AQI delivered to their email inbox?
- EnviroFlash is a free service that sends emails about your daily air quality forecast
- More information about this free service is at www.enviroflash.info
Where can I get more information?
- Along with air quality forecasts and real-time air quality information, EPA's Web site (www.airnow.gov) includes patient information materials physicians can use in their practice. The site also includes a page for elementary school-aged children that includes a guide for teachers.
- Many of these materials are also available in the Patient Education section of this course.