Depending on the composition of the aspirate, three complications have been described:
Chemical pneumonitis may develop whose severity depends on the pH value and quantity of aspirate. The two lung changes after acid aspiration are:  a) direct toxic damage to the respiratory epithelium resulting in interstitial pulmonary edema and b) a few hours later, inflammatory response with production of cytokines, neutrophil infiltration, and macrophage activation. Oxygen-free radicals are generated which, in turn, lead to further lung damage. Patients may remain asymptomatic after acid aspiration. Others may develop dyspnea, pleuritic chest pain, cough, fever, bloody or frothy sputum, and respiratory failure. Aspiration pneumonia may develop. The third complication occurs after inhalation of particulate matter that obstructs airways. The patients will have sudden arterial hypoxemia with development of lung atelectasis. Location
The location is often gravity dependent, and depends on the persons position. Generally, the right middle and lower lung lobes are the most common sites affected, due to the larger caliber and more vertical orientation of the right mainstem bronchus. People who aspirate while standing can have bilateral lower lung lobe infiltrates. The right upper lobe is a common area of consolidation, where liquids accumulate in a particular region of the lung, in alcoholics who aspirate in the supine position. Diagnosis
Evaluation of aspiration is generally performed with a video fluoroscopic swallowing study involving radiologic evaluation of the swallowing mechanism via challenges with liquid and solid food consistencies.
Bacterial colonization: Poor oral hygiene can result in colonization of the mouth with excessive amounts of bacteria, which is linked to increased incidence of aspiration pneumonia. Ethnicity: Asians diagnosed with aspiration pneumonia have a lower risk of death compared to other ethnic groups while African Americans and whites share a relatively similar risk of death. Hispanics have a lower risk of death than non-Hispanics. Others: Age, male gender, diabetes mellitus, malnutrition, use of antipsychotic drugs, proton pump inhibitors, and angiotensin-converting enzyme inhibitors. Residence in an institutional setting, prolonged hospitalization or surgical procedures, gastric tube feeding, mechanical airway interventions, immunocompromised, history of smoking, antibiotic therapy, advanced age, reduced pulmonary clearance, diminished cough reflex, disrupted normal mucosal barrier, impaired mucociliary clearance, alter cellular and humoral immunity, obstruction of the airways, and damaged lung tissue. Bacteria
Bacteria involved in aspiration pneumonia may be either aerobic or anaerobic. Common aerobic bacteria involved include:

Streptococcus pneumoniae
Staphylococcus aureus
Haemophilus influenzae
Pseudomonas aeruginosa
Klebsiella: often seen in aspiration lobar pneumonia in alcoholicsAnaerobic bacteria also play a key role in the pathogenesis of aspiration pneumonia. They make up the majority of normal oral flora and the presence of putrid fluid in the lungs is highly suggestive of aspiration pneumonia secondary to an anaerobic organism. While it is difficult to confirm the presence of anaerobes through cultures, the treatment of aspiration pneumonia typically includes anaerobic coverage regardless. Potential anaerobic bacteria are as follows:

Bacteroides
Prevotella
Fusobacterium
Peptostreptococcus

Pathophysiology
Aspiration is defined as inhalation of oropharyngeal or gastric contents into the pulmonary tree.