There are two primary environmental aspects to be considered when selecting a species of lumber to build your deck.
In the first place, if you're searching for an exotic softwood species, it is likely to require treatment using an acid-based preservative to ensure the necessary durability for the longevity of service.
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Sometimes, there have been concerns raised regarding the risk of harm from the chemicals used, particularly the risk of chemicals that are leaching from the wood. This is admittedly a gray space with contradicting "evidence" that is presented to support claims one in one direction or another.
One of the most common chemical preservatives used in the past has been CCA (copper-chrome-arsenic) and although such products have been used for decades, some countries or regions have now taken steps to ban the use of CCA, at least in public spaces.
In recent times, a range of wood preservation alternatives is available that claim to have minimal or no risk of harm. But whether these products offer the same protection over time of the wood as CCA is yet to be determined.
However, the most important environmental aspect to be taken into consideration is that of forest destruction and making sure that any wood comes by forests properly managed with a long-term sustainable perspective with no impact on the surrounding environmental conditions and in compliance with the laws of the country of the source.
In the past rainforests in the tropical trees have been destroyed in order to earn quick, short-term profits that have devastating long-term implications.