PARIS (Reuters) - The European Commission has decided to sue Portugal for its poor air quality caused by high levels of nitrogen dioxide, it said on Friday.

       "Portugal has continually and persistently exceeded the annual nitrogen dioxide limit value in three air quality zones (...)", the Commission said in a statement, adding it was referring Portugal to the European Court of Justice.

       "Full implementation of the air quality standards enshrined in EU legislation is key to effectively protect human health and safeguard the natural environment".

       (Reporting by Marine Strauss and Benoit Van Overstraeten; Editing by Sabine Siebold)