Alkaline copper quaternary (ACQ) is a wood preservative that helps protect against decay and insect damage. It contains copper, which acts as a fungicide, and quaternary ammonium compounds that provide additional protection. ACQ is often used in outdoor applications, such as decks and fences, due to its effectiveness and lower environmental impact compared to traditional preservatives like chromated copper arsenate (CCA). ACQ-treated wood is safe for various uses, including residential and commercial projects. It is important to note that while ACQ is less toxic, it can still corrode certain metals, so proper precautions should be taken when using it in conjunction with materials like aluminum or steel.