Air pollution is a major cause of cardiorespiratory morbidity and mortality. The exact components of air pollution that underlie the cardiovascular effects are not yet known, but combustion-derived particulate matter is suspected to be the major cause. Epidemiological studies have shown that exposure to air pollution causes exacerbation of existing cardiorespiratory conditions leading to increased hospital admissions and death. The investigators have recently conducted a series of controlled exposure studies to urban particulate matter and diesel exhaust in healthy volunteers and patients with coronary heart disease. The investigators found that controlled exposure to dilute diesel exhaust in patients with prior myocardial infarction induced asymptomatic myocardial ischaemia with an increase in electrocardiographic measures of myocardial ischaemia. Whilst important, further questions remain: (i) does air pollution exposure exacerbate ischaemia and reduce exercise tolerance in patients with symptomatic angina pectoris, (ii) do "real world" exposures as encountered in the urban environment of major cities have similar effects, and (iii) can a simple face mask intervention to reduce exposure to particulate air pollution improve health outcomes in patients with coronary heart disease?

100 patients with stable angina pectoris will be recruited from the HPS-THRIVE cohort or from the cardiology outpatient department at the Fuwai Hospital in Beijing. Patients will attend the department first thing in the morning on two occasions, separated by at least 1 week. All subjects will be fitted with a 12-lead Holter monitor and 24-hour ambulatory blood pressure monitor. They will be issued with a diary to record their activities and symptoms.

Patients will be randomised to wear a simple facemask for one of their visits (3M Dust Respirator 8812). When randomised to wear a mask, subjects will be asked to wear the mask as much as possible in the 24 hours prior to the study day and for the 24 hours of the study day (48 hours in total). Subjects will be instructed to wear the mask at all times when outdoors, and as much as possible when indoors.

On both visits, patients will be asked to walk along a prespecified route for 2 hours in a city centre location in Beijing. During this period patients will walk at their own pace for 15 mins and rest for 5 mins for a total of 120 mins (6 cycles). After the walk, subjects will return to the department and the pollution equipment switched off. A formal BRUCE protocol exercise stress test will be performed as soon as possible after finishing the 2-hour walk, and blood tests will be taken just prior to the exercise stress test (10-20mls in total). Subjects will be asked to wear the ambulatory blood pressure monitor and the Holter ECG monitor for a total of 24 hours, returning the following day to have this removed. Personal exposure to air pollution will be determined during the 2 hour walk with a series of monitors contained within a small backpack.

Inclusion Criteria:

• Stable coronary heart disease
• Symptoms of angina pectoris
• Evidence of coronary heart disease by angiography or by exercise stress testing

Exclusion Criteria:

• Current smokers
• History of arrhythmia
• Severe 3 vessel coronary heart disease or left main stem stenosis that has not been revascularised
• Resting conduction abnormality
• Digoxin therapy
• Uncontrolled hypertension
• Renal failure
• Hepatic failure
• Acute coronary syndrome or unstable symptoms within 3 months
• Significant occupation exposure to air pollution