Asthma is a chronic respiratory disease characterized by recurrent episodes of shortness of breath and wheezing. Some causes and triggers are common to all people with asthma, and some are more unique. Although the major causes of asthma are not fully understood, the greatest risk factors for developing asthma are inhaled asthma triggers. These include: indoor allergens (e.g., household dust mites in bedding, carpet and plush furniture, pollution, and pet dander); outdoor allergens (e.g., pollen and mold); tobacco smoke; and chemical irritants in the workplace. Other triggers may include cold air, extreme emotional arousal such as anger or fear, and physical activity. In some people, asthma may even be triggered by certain medications, such as aspirin and other nonsteroidal anti-inflammatory drugs and blockers (used to treat high blood pressure, heart disease, and migraines). Urbanization has also been associated with an increase in asthma, although the exact nature of this relationship remains unclear. According to WHO estimates, 300 million people worldwide suffer from asthma, and 255,000 people died from asthma in 2005. Although asthma cannot be cured, proper management can control the disease and enable people to improve their quality of life. In addition, some children with mild asthma experience a gradual disappearance of their symptoms as they get older.