• Waste products such as plastic bags and tin cans can end up hurting sea life.a can left on a sea shore image by yelnami from Fotolia.com
  
  Seas all around the world are contaminated with pollutants, which pose a threat to marine flora and fauna and also to human beings who swim in the water. These pollutants come from a variety of sources, but most are the results of human activity. According to the World Wide Fund for Nature website, 80 percent of the pollution found in marine environments result from human activities on land.
  
  

Product Waste

• Humans dump solid waste that finds its way into the sea in the form of packaging, plastic bags and even items of clothing. This waste then washes up on the shore, where it becomes a form of pollution on beaches and in salt marshes. Waste products such as plastic bags also threaten animals that might eat the products by mistake or else injure themselves when encountering them. Waste items often decompose at a gradual rate, which means that plastic packaging and containers stick around for a long time.
  
  

Sewage Water

• Sewage water, if discharged inappropriately, can find its way into the sea, where it can lead to the appearance of tiny plants, thanks to the level of nitrogen and phosphorus found in raw sewage. This process is called eutrophication, and it's dangerous because the subsequent decay of the plants leads to a reduction in the water's oxygen and is harmful for marine life.
  
  Sewage is also a threat to humans who bathe in the sea. This problem is worse around coastal areas with beaches, where individuals are likely to participate in sports such as surfing. As the Surfers Against Sewage website points out, humans using beaches for recreation are particularly likely to become ill due to contact with sewage-based pollutants.
  
  

Chemicals

• In the past, people have dumped chemicals into the sea when no longer required, leading to everything from radioactive materials to pesticides ending up in marine environments, where they have contaminated local life forms. As of 2010, many of these chemicals can't legally be dumped into the sea, but chemicals stick around for a long time, remaining toxic, while containers carrying chemicals might leak during transit.
  
  

Metals

• Metals have the potential to become dangerous if dumped into the sea, since many metals such as tin and lead become toxic when oxygenated by water. These metals are then dangerous to marine life such as shellfish. Metals enter the sea when they're dumped in large quantities by nearby industries. Products such as tributyltin, which was used in paint on ships before 1988, have also deposited metals into the sea.