Byssinosis is an occupational lung disease caused by inhalation of cotton or jute dust in inadequately ventilated working environments and can develop over time with repeated exposure. Byssinosis commonly occurs in textile workers who are employed in yarn and fabric manufacture industries. It is now thought that the cotton dust directly causes the disease and some believe that the causative agents are endotoxins that come from the cell walls of gram-negative bacteria that grow on the cotton. Although bacterial endotoxin is a likely cause, the absence of similar symptoms in workers in other industries exposed to endotoxins makes this uncertain. Current smokers are also at risk for developing byssinosis or having complications relating to byssinosis.

Of the 81 byssinosis-related fatalities reported in the United States between 1990 and 1999, 48% included an occupation in the yarn, thread, and fabric industry on their death certificate. This disease often occurred in the times of the Industrial Revolution. Most commonly young girls working in mills or other textile factories would be affected by this disease. In the United States, from 1996 to 2005, North Carolina accounted for about 37% of all deaths caused by byssinosis, with 31, followed by South Carolina (8) and Georgia (7).

There is a lack of information regarding the prevalence and impact of byssinosis in low and middle income countries (LMIC) despite the fact that of the 25 million tons of cotton produced worldwide, about two thirds of this production comes from LMICs like India, Pakistan, and China. Many textile mills and fiber producing factories located in LMICs have high rates of chronic respiratory disease caused by byssinosis.

The term "brown lung" is a misnomer, as the lungs of affected individuals are not brown. Byssinosis is also referred to as cotton worker's lung, mill fever, brown lung disease, and Monday fever.