Acute Interstitial Pneumonitis can result from many different irritants in the lungs and usually is resolved in under a month. Chemical Pneumonitis is caused by toxic substances reaching the lower airways of the bronchial tree. This causes a chemical burn and severe inflammation. (oxford)
Radiation Pneumonitis, also known as Radiation Induced Lung Injury, describes the initial damage done to the lung tissue by ionization radiation. Radiation, used to treat cancer, can cause pneumonitis when applied to the chest or full body. Radiation pneumonitis occurs in approximately 30% of advanced lung cancer patients treated with radiation therapy. Aspiration pneumonitis is caused by a chemical inhalation of harmful gastric contents which include causes such as:
Aspiration due to a drug overdose
A lung injury after the inhalation of habitual gastric contents. The development of colonized oropharyngeal material after inhalation. Bacteria entering the lungs

Treatment
Typical treatment for pneumonitis includes conservative use of corticosteroids such as a short course of oral prednisone or methylprednisolone. Inhaled corticosteroids such as fluticasone or budesonide may also be effective for reducing inflammation and preventing re-inflammation on a chronic level by suppressing inflammatory processes that may be triggered by environmental exposures such as allergens. Severe cases of pneumonitis may require corticosteroids and oxygen therapy, as well as elimination of exposure to known irritants.Corticosteroid dose and treatment duration vary from case to case.
Any particles that are smaller than 5 microns can enter the alveoli of the lungs. These tiny air sacs facilitate the passage of oxygen from inhaled air to the bloodstream. In the case of pneumonitis, it is more difficult for this exchange of oxygen to occur since irritants have caused inflammation of the alveoli. Due to the lack of a definitive determination of a single irritant causing pneumonitis, there are several possible causes. Viral infection. Measles can cause severe pneumonitis, and ribavirin has been proposed as a possible treatment. CMV is another cause. Pneumonia
Radiation therapy
Inhaling chemicals, such as sodium hydroxide
Interstitial lung disease
Sepsis
Adverse reaction to medications
Hypersensitivity to inhaled agents
Inhalation of spores of some species of mushroom (bronchoalveolar allergic syndrome)
Mercury exposure
Smoking
Overexposure to chlorine
Bronchial obstruction (obstructive pneumonitis or post-obstructive pneumonitis)
Ascariasis (during parasite migration)
Aspirin overdose, some antibiotics, and chemotherapy drugs
“Farmer’s lung” and “hot tub lung” are common names for types of hypersensitivity pneumonitis that result from exposure to some types of thermophilic actinomyces, mycobacteria and molds. Avian proteins in bird feces and feathers 
Whole body or chest radiation therapy used for cancer treatment

Symptoms
Physical manifestations of Pneumonitis range from mild cold-like symptoms to respiratory failure. Most frequently, those with pneumonitis experience shortness of breath, and sometimes a dry cough.