The primary risk factor for COPD globally is tobacco smoking. Of those who smoke, about 20% will get COPD, and of those who are lifelong smokers, about half will get COPD. In the United States and United Kingdom, of those with COPD, 80–95% are either current smokers or previously smoked. The likelihood of developing COPD increases with the total smoke exposure. Additionally, women are more susceptible to the harmful effects of smoke than men. In nonsmokers, secondhand smoke is the cause of about 20% of cases. Other types of smoke, such as, marijuana, cigar, and water-pipe smoke, also confer a risk. Water-pipe smoke appears to be as harmful as smoking cigarettes. Problems from marijuana smoke may only be with heavy use. Women who smoke during pregnancy may increase the risk of COPD in their child. For the same amount of cigarette smoking, women have a higher risk of COPD than men.

Health care professionals are at risk of occupational influenza exposure; during a pandemic influenza, anyone in a close environment is at risk, including those in an office environment.

Tobacco smoke is a known carcinogen. Workers in the hospitality industry may be exposed to tobacco smoke in the workplace, especially in environments like casinos and bars/restaurants.

Intense and prolonged exposure to workplace dusts, chemicals, and fumes increases the risk of COPD in both smokers and nonsmokers. Workplace exposures are believed to be the cause in 10–20% of cases. In the United States, they are believed to be related to more than 30% of cases among those who have never smoked and probably represent a greater risk in countries without sufficient regulations.

A number of industries and sources have been implicated, including high levels of dust in coal mining, gold mining, and the cotton textile industry, occupations involving cadmium and isocyanates, and fumes from welding. Working in agriculture is also a risk. In some professions, the risks have been estimated as equivalent to that of one-half to two packs of cigarettes a day. Silica dust and fiberglass dust exposure can also lead to COPD, with the risk unrelated to that for silicosis. The negative effects of dust exposure and cigarette smoke exposure appear to be additive or possibly more than additive.

Pneumonia is due to infections caused primarily by bacteria or viruses and less commonly by fungi and parasites. Although there are more than 100 strains of infectious agents identified, only a few are responsible for the majority of the cases. Mixed infections with both viruses and bacteria may occur in up to 45% of infections in children and 15% of infections in adults. A causative agent may not be isolated in approximately half of cases despite careful testing.

The term "pneumonia" is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as pneumonitis.

Conditions and risk factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, asthma, chronic kidney disease, and liver disease. The use of acid-suppressing medications—such as proton-pump inhibitors or H2 blockers—is associated with an increased risk of pneumonia. The risk is also increased in old age.

Several studies found that healthcare-associated pneumonia is the second most common type of pneumonia, occurring less commonly than community-acquired pneumonia but more frequently than hospital-acquired pneumonia and ventilator-associated pneumonia. In a recent observational study, the rates for CAP, HCAP and HAP were 60%, 25% and 15% respectively. Patients with HCAP are older and more commonly have simultaneous health problems (such as previous stroke, heart failure and diabetes).

The number of residents in long term care facilities is expected to rise dramatically over the next 30 years. These older adults are known to develop pneumonia 10 times more than their community-dwelling peers, and hospital admittance rates are 30 times higher.

Bacteria are the most common cause of community-acquired pneumonia (CAP), with "Streptococcus pneumoniae" isolated in nearly 50% of cases. Other commonly isolated bacteria include "Haemophilus influenzae" in 20%, "Chlamydophila pneumoniae" in 13%, and "Mycoplasma pneumoniae" in 3% of cases; "Staphylococcus aureus"; "Moraxella catarrhalis"; "Legionella pneumophila" and Gram-negative bacilli. A number of drug-resistant versions of the above infections are becoming more common, including drug-resistant "Streptococcus pneumoniae" (DRSP) and methicillin-resistant Staphylococcus aureus (MRSA).

The spreading of organisms is facilitated when risk factors are present. Alcoholism is associated with "Streptococcus pneumoniae", anaerobic organisms, and "Mycobacterium tuberculosis"; smoking facilitates the effects of "Streptococcus pneumoniae", "Haemophilus influenzae", "Moraxella catarrhalis", and "Legionella pneumophila". Exposure to birds is associated with "Chlamydia psittaci"; farm animals with "Coxiella burnetti"; aspiration of stomach contents with anaerobic organisms; and cystic fibrosis with "Pseudomonas aeruginosa" and "Staphylococcus aureus". "Streptococcus pneumoniae" is more common in the winter, and should be suspected in persons aspirating a large amount of anaerobic organisms.

Acute bronchitis is one of the most common diseases. About 5% of adults are affected and about 6% of children have at least one episode a year. It occurs more often in the winter. More than 10 million people in the United States visit a doctor each year for this condition with about 70% receiving antibiotics which are mostly not needed. There are efforts to decrease the use of antibiotics in acute bronchitis.

Respiratory disease is a common and significant cause of illness and death around the world. In the US, approximately 1 billion "common colds" occur each year. A study found that in 2010, there were approximately 6.8 million emergency department visits for respiratory disorders in the U.S. for patients under the age of 18. In 2012, respiratory conditions were the most frequent reasons for hospital stays among children.

In the UK, approximately 1 in 7 individuals are affected by some form of chronic lung disease, most commonly chronic obstructive pulmonary disease, which includes asthma, chronic bronchitis and emphysema.

Respiratory diseases (including lung cancer) are responsible for over 10% of hospitalizations and over 16% of deaths in Canada.

In 2011, respiratory disease with ventilator support accounted for 93.3% of ICU utilization in the United States.

Eosinophilic pneumonia is a rare disease. Parasitic causes are most common in geographic areas where each parasite is endemic. AEP can occur at any age, even in previously healthy children, though most patients are between 20 and 40 years of age. Men are affected approximately twice as frequently as women. AEP has been associated with smoking. CEP occurs more frequently in women than men and does not appear to be related to smoking. An association with radiation for breast cancer has been described.

Among the factors contributing to contracting HAP are mechanical ventilation (ventilator-associated pneumonia), old age, decreased filtration of inspired air, intrinsic respiratory, neurologic, or other disease states that result in respiratory tract obstruction, trauma, (abdominal) surgery, medications, diminished lung volumes, or decreased clearance of secretions may diminish the defenses of the lung. Also, poor hand-washing and inadequate disinfection of respiratory devices cause cross-infection and are important factors.

Dust pneumonia describes disorders caused by excessive exposure to dust storms, particularly during the Dust Bowl in the United States. A form of pneumonia, dust pneumonia results when the lungs are filled with dust, inflaming the alveoli.

Symptoms of dust pneumonia include high fever, chest pain, difficulty in breathing, and coughing. With dust pneumonia, dust settles all the way into the alveoli of the lungs, stopping the cilia from moving and preventing the lungs from ever clearing themselves.

People who had dust pneumonia often died. There are no official death rates published for the Great Plains in the 1930s. In 1935, dozens of people died in Kansas from dust pneumonia. Red Cross volunteers made and distributed thousands of dust masks, although some farmers and other people in the affected areas refused to wear them.

Most cases of chronic bronchitis are caused by smoking cigarettes or other forms of tobacco. Additionally, chronic inhalation of air pollution or irritating fumes or dust from hazardous exposures in occupations such as coal mining, grain handling, textile manufacturing, livestock farming, and metal moulding may also be a risk factor for the development of chronic bronchitis. Protracted bacterial bronchitis is usually caused by "Streptococcus pneumoniae", "Non-typable Haemophilus influenzae", or "Moraxella catarrhalis".

Silicosis resulted in 46,000 deaths in 2013 down from 55,000 deaths in 1990.

Mycoplasma is found more often in younger than in older people.

Older people are more often infected by Legionella.

When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial cause and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS)

and measles.

As the lungs tend to be vulnerable organs due to their exposure to harmful particles in the air, several things can cause an acute exacerbation of COPD:

- Respiratory infection, being responsible for approximately half of COPD exacerbations. Approximately half of these are due to viral infections and another half appears to be caused by bacterial infections. Common bacterial pathogens of acute exacerbations include "Haemophilus influenzae", "Streptococcus pneumoniae" and "Moraxella catarrhalis". Less common bacterial pathogens include "Chlamydia pneumoniae" and "MRSA". Pathogens seen more frequently in patients with impaired lung function (FEV<35% of predicted) include "Haemophilus parainfluenzae" (after repeated use of antibiotics), "Mycoplasma pneumoniae" and gram-negative, opportunistic pathogens like "Pseudomonas aeruginosa" and "Klebsiella pneumoniae".

- Allergens, e.g., pollens, wood or cigarette smoke, pollution

- Toxins, including a variety of different chemicals

- Air pollution

- Failing to follow a drug therapy program, e.g. improper use of an inhaler

In one-third of all COPD exacerbation cases, the cause cannot be identified.

Affected workers should be offered alternative employment. Continued exposure leads to development of persistent symptoms and progressive decline in FEV1.

The following are precautionary measures that can be taken to avoid the spread of bagassosis:

1. Dust control-prevention /suppression of dust such as wet process, enclosed apparatus, exhaust ventilation etc. should be used

2. Personal protection- masks/ respirators

3. Medical control- initial medical examination & periodical checkups of workers

4. Bagasse control- keep moisture content above 20% and spray bagasse with 2% propionic acid

Eosinophilic pneumonia due to cancer or parasitic infection carries a prognosis related to the underlying illness. AEP and CEP, however, have very little associated mortality as long as intensive care is available and treatment with corticosteroids is given. CEP often relapses when prednisone is discontinued; therefore, some people with CEP require lifelong therapy. Chronic prednisone is associated with many side effects, including increased infections, weakened bones, stomach ulcers, and changes in appearance.

Pulmonary diseases may also impact newborns, such as pulmonary hyperplasia, pulmonary interstitial emphysema (usually preterm births), and infant respiratory distress syndrome,

Since the start of the AIDS epidemic, PCP has been closely associated with AIDS. Because it only occurs in an immunocompromised host, it may be the first clue to a new AIDS diagnosis if the patient has no other reason to be immunocompromised (e.g. taking immunosuppressive drugs for organ transplant). An unusual rise in the number of PCP cases in North America, noticed when physicians began requesting large quantities of the rarely used antibiotic pentamidine, was the first clue to the existence of AIDS in the early 1980s.

Prior to the development of more effective treatments, PCP was a common and rapid cause of death in persons living with AIDS. Much of the incidence of PCP has been reduced by instituting a standard practice of using oral co-trimoxazole (Bactrim / Septra) to prevent the disease in people with CD4 counts less than 200/μL. In populations that do not have access to preventive treatment, PCP continues to be a major cause of death in AIDS.

Dust pneumonia was featured in the work of several musicians and artists of the day, such as Woody Guthrie's song "Dust Pneumonia Blues".

Sources of such lipids could be either exogenous or endogenous.

Exogenous: from outside the body. For example, inhaled nose drops with an oil base, or accidental inhalation of cosmetic oil. Amiodarone is an anti-arrythmic known to cause this condition. Oil pulling has also been shown to be a cause. At risk populations include the elderly, developmentally delayed or persons with gastroesophageal reflux. Switching to water-soluble alternatives may be helpful in some situations.

Endogenous: from the body itself, for example, when an airway is obstructed, it is often the case that distal to the obstruction, lipid-laden macrophages (foamy macrophages) and giant cells fill the lumen of the disconnected airspace.

Acute exacerbations can be partially prevented. Some infections can be prevented by vaccination against pathogens such as influenza and "Streptococcus pneumoniae". Regular medication use can prevent some COPD exacerbations; long acting beta-adrenoceptor agonists (LABAs), long-acting anticholinergics, inhaled corticosteroids and low-dose theophylline have all been shown to reduce the frequency of COPD exacerbations. Other methods of prevention include:

- Smoking cessation and avoiding dust, passive smoking, and other inhaled irritants

- Yearly influenza and 5-year pneumococcal vaccinations

- Regular exercise, appropriate rest, and healthy nutrition

- Avoiding people currently infected with e.g. cold and influenza

- Maintaining good fluid intake and humidifying the home, in order to help reduce the formation of thick sputum and chest congestion.