Houston's predominant air pollution problem. In 2011, Houston was ranked as the 17th most polluted city in the US, according to the American Lung Association. A 2007 assessment found the following twelve air pollutants to be definite risks to health in Houston: ozone - respiratory and cardiovascular effects particulates less than 2.5 μm in diameter (PM2.5) - respiratory and cardiopulmonary effects diesel particulate matter (DPM) - cancer 1,3-butadiene - cancer and reproductive effects hexavalent chromium - cancer benzene - cancer ethylene dibromide - cancer acrylonitrile - cancer formaldehyde - cancer and respiratory effects acrolein - respiratory effects chlorine - respiratory effects hexamethylene diisocyanate - pulmonary and respiratory effects The State of Texas concluded that, since 2000, the Manchester neighborhood in eastern Houston had the highest annual averages of 1,3-butadiene of any area in Texas. Houston's air quality has often been compared to Los Angeles and Beijing. Houston has introduced many programs since the 2000 federal order to reduce air pollution in the city. The most notable project was the METRORail light rail system constructed in 2004. The light rail system was designed to encourage Houstonians to utilize public transportation instead of their automobiles. Pollution rankings from the EPA and ALA are administered in terms of peak-based standards, focusing strictly on the worst days of the year; on average, ozone levels in Houston are lower than those seen in many other cities of the country, due to dominant winds off the Gulf that ensure clean, marine air. Water pollution Houston has also seen recent improvements to the city's waterways. The banks of Buffalo Bayou have been cleaned of garbage and have been turned into jogging trails and parks. Since the mid-1990s, Houston has seen a great increase in wildlife along the bayou due to many successful cleaning attempts. The Port of Houston has not seen any major cleaning attempts. Notes References External links Weather Averages for Houston Comparison with month average and actual temps Houston Climate of Texas Houston