However, increasingly and especially as concentrations of the classical combustion derived pollutants decline it may not be these that are most critical for objects on display.
The portion of rivers and streams "severely impaired" by pollutants had declined to just 1percentt in 1998 from 5percentt in 1993.
Most air pollutants have declined sharply in recent decades, and the amount of forest land hasn't been shrinking at all — it's been fairly stable since 1920 and has actually grown in the last decade.
In the last two decades, emissions of the most common air pollutants have declined by 41percentt, while the Gross National Product (GDP) has increased by 64percentt.
Although ambient levels of the major criteria pollutants have declined in the United States over the last 30 years, much less is known about exposure to many of the 189 hazardous air pollutants identified in the 1990 Clean Air Act Amendments (Clean Air Act Amendments 1990).
It also required that total emissions of mercury, sulfur dioxide and other pollutants decline by 10percentt in the same period.
Several studies from the last decade linked the trace pollutants to declining sperm counts, infertility and various forms of cancer in humans, as well as genital malformation in wildlife.
Unfortunately, humans are not so kind to orcas: pollutants and declining fish stocks, especially salmon in the North Pacific, are thought to be causing population declines in several regions.
Importantly, measured concentrations of several pollutants in California declined substantially from 2002 through 2007; annual average PM2.5, organic carbon (OC), and nitrates decreased by around 30% each.
By several indicators, the emissions of a number of pollutants have, in fact, declined over the last several years.
