Sulfur dioxide is a colorless, odorless, water-soluble gas found commonly in the atmosphere of industrial countries. This gas is produced when fossil fuel is burned, a necessary part of our modern lifestyle. Exposure to high levels of SO2 gas (10 to 50 PPM) can cause irritation of the eyes, nose and throat as well as associated coughing and chest tightness. The state and federal governments have laws that control SO2 emissions. Controlling these emissions has resulted in lower exposure levels.

Today, levels found in most communities do not result in noticeable symptoms for most of the population. However, very low levels of SO2 may affect some asthmatics. The information below is intended to give you some basic information about asthma and how SO2 may rarely affect this condition.

Asthma is a disease that affects the air passages in the lungs. People with asthma can develop shortness of breath, chest tightness, cough and wheezing following exposure to a variety of non-specific triggers. These symptoms are caused by narrowing of the airways, which occurs as a response to various stimuli. Symptoms can occur suddenly but typically only last a few minutes to a few hours. Between these episodes, lung function usually returns to normal and does not interfere with daily activities.

Approximately half of the cases of asthma develop before age ten and another third occur before age 40. Most individuals with asthma seek medical attention at some point for relief of recurrent symptoms and thus have been formally diagnosed. However, some individuals with mild disease do not seek medical attention and may not be aware that they have this condition.

Individuals with asthma can experience symptoms following a wide variety of exposures or events. Interestingly, lung function can change significantly throughout the day and many people with asthma experience increased symptoms at night and/or in the early morning. Other common triggers are: upper respiratory infections, allergens, exercise, cold air, low humidity, certain medications, food additives, fragrances and some air pollutants like SO2.

SO2 is a highly water-soluble gas that, when inhaled, is almost entirely absorbed by the mucous membranes in the nose and upper airways. These membranes effectively reduce the level of inspired SO2 to approximately 2% of the initial inhaled concentration, which helps to protect lower airways from significant exposure. During situations like exercise that increase the breathing rate and the volume of inspired air, a greater amount of SO2 reaches the lower airways.

Asthmatic individuals may react to these seemingly minimal increases in SO2 reaching lower airways whereas people without asthma do not. This is because people with asthma have a tendency to develop an exaggerated airway response to a wide variety of non-specific stimuli. This reaction can take the form of chest tightness, shortness of breath, cough or wheezing and may lead to disruption of activity, and/or the need for medication.

Please see the chart below.

An asthmatic's response to SO2 is highly variable and cannot be accurately predicted based on either the severity or frequency of their previous asthmatic episodes. For example, certain individuals who have asthma classified as moderate/severe may not react to very low levels of SO2. Conversely, some mild asthmatics have been known to react to low levels of SO2.

A previous reaction to SO2 appears to be the best predictor of the likelihood of future reactions.

The effects of significant SO2 exposure can be seen after five minutes of exposure while exercising. Side effects generally do not increase significantly beyond 10 minutes of exposure and usually resolve completely within 30 minutes to 1 hour.

• Follow your doctor's treatment advice and take medication as directed. Limit your exposure to the triggers you can control.
• If your asthma is worsened by SO2, limit physical exertion when SO2 levels in your area are elevated (see table below).
• Ask your doctor about the possibility of using a beta agonist. Beta agonists are drugs that are commonly used to treat asthma. Use of a beta agonist before exposure can prevent symptoms from developing. If used after exposure it can greatly decrease existing symptoms.

1. Your personal health care provider

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