Portugal has 2 million pigs, of which 45% are in Ribatejo and Leiria. These two areas concentrate 1/3 of the total pigs in the region, the equivalent of 15% of the national total. The farms are highly specialized and raise an average of 200 animals each. The second most important region of production is Alentejo, with 25% of the national total, and the third is Beira Coast, with 21% of pigs.

Discussions on the existence of pollution in the Lis River began in 1971 - the year when the first study on the causes of the pollution was carried out. Several causes were listed, including discharges of sewage and waste from slaughterhouses and swine barns. In the late 1970s, the pollution in the Lis River worsened and became publicly known.

There were repeated episodes where pollution from the discharges from pig farms reached dangerous levels, killing hundreds of fish. Sewage from the farms was considered the main cause for the impacts.

In June 1987, a group of inhabitants from the Carvide parish gathered 365 signatures on a petition against the pollution of the Lis River. In 1988, the press referred to the situation of the Lis as the "Suinobyl Disaster", a reference to the accident at the Chernobyl nuclear power station. In the 1990s, due to the increase in pollution and popular discontent, the Association for Environment Defense and Heritage of Leiria (OIKOS) was founded.

In 2002 and 2003, the most significant occurrences related to pollution in the Lis River was when the water supply had to be cut off for several days in the city of Leiria and bathing on the Vieira beach, on the coast of Leiria, was banned due to pollutant discharges. These incidences led to local residents to create the Comissão de Ambiente e Defesa da Ribeira dos Milagres (CADRM, or the Environment and Defense of the Ribeira dos Milagres Committee). Recilis, a company mostly composed by pig farmers in the region, was also created to deal with and treat the activity of effluents.

In the following years, the discharges continued and were denounced by the population, CADRM, and Quercus, a national environmental association. The government announced plans to build a treatment plant for effluents from pig farms, which was to be operational by 2008. However, it did not follow through with them. Complaints of the discharge of effluents into the river continued and, in 2008, OIKOS announced the results of a study to monitor the water in the Lis River basin that had begun in 1990: the level of fecal pollution was disturbing, and the most problematic points were the Ribeira do Sirol, Ribeira dos Milagres, and Arrabalde Bridge area (Leiria).

In 2003, the EU passed specialized legislation on the protection of pigs that regulated their living conditions and feeding, which was to be implemented by all European countries by January 1, 2013. It was estimated at the time that the necessary changes would require an investment of 200 euros/animal. By mid-2013, only 75% of farms had complied with the new legislation. According to the president of the Portuguese Federation of Pig Farmers (FPAS in Portuguese), the reason for the failure to implement the legislation is the high cost to swine farmers, estimated at a total of 50 million euros. Even so, the expectation that by September 2013, 100% of farms would have complied with the regulations. In the meantime, Portugal is required to pay fines to the EU for breaking the law.

In April 2010, Quercus held a protest near Ribeira dos Milagres to demand the monitoring of and solutions to the problem of swine farm pollution. Discharges were once again reported. In 2013, the government signed a protocol for the construction of a treatment plant in the region, which was expected to be completed in two years’ time. In June of that year, it was decide that the wastewater treatment station ‘ETES, Estação Tratamento Esgotos Sanitários’ would be built in Leiria. The works were to be concluded by June 2018.

CADRM organized a large demonstration on May 22, 2017 for the clean-up of the Lis River after analyses on the water of the river that flows through the city of Leiria revealed that the pollution continues to get worse and the ETES has still not been built.

In other locations in Portugal, people suffering from the effects of pollution from hog farms have also organized mobilizations. In Póvoas and Rio Maior (Santarém), pollution caused by swine raising generated popular protests and was discussed in the parliament and in Castro Verde (Beja). In the 2000s, attempts to expand hog farming in Castro Verde were met with strong resistance from the local population, who organized a petition to stop the company from obtaining a license.

Many are the hog farms owners operate in the area.