pressure of up to (but more commonly 200 psi) to form the plywood panel. The panel can then be patched, have minor surface defects such as splits or small knot holes filled, re-sized, sanded or otherwise refinished, depending on the market for which it is intended. Plywood for indoor use generally uses the less expensive urea-formaldehyde glue, which has limited water resistance, while outdoor and marine-grade plywood are designed to withstand moisture, and use a water-resistant resorcinol-formaldehyde or phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity. The adhesives used in plywood have become a point of concern. Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations. As a result, many manufacturers are turning to low formaldehyde-emitting glue systems, denoted by an "E" rating. Plywood produced to "E0" has effectively zero formaldehyde emissions. In addition to the glues being brought to the forefront, the wood resources themselves are becoming the focus of manufacturers, due in part to energy conservation, as well as concern for natural resources. There are several certifications available to manufacturers who participate in these programs. Programme for the Endorsement of Forest Certification (PEFC) Forest Stewardship Council (FSC), Leadership in Energy and Environmental Design (LEED), Sustainable Forestry Initiative (SFI), and Greenguard are all certification programs that ensure that production and construction practices are sustainable. Many of these programs offer tax benefits to both the manufacturer and the end user. Sizes The most commonly used thickness range is from . The sizes of the most commonly used plywood sheets are 4 x 8 feet (1220 x 2440 mm) which was first used by the Portland Manufacturing Company, who developed what we know of as modern veneer core plywood for the 1905 Portland World Fair. A common metric size for a sheet of plywood is 1200 x 2400 mm. is also a common European size for Baltic birch ply, and aircraft ply. Sizes on specialised plywood for concrete-forming can range from , and a multitude of formats exist, though 15 × (19/32in × 2 ft-6in × 4 ft-11in) is very commonly used. Aircraft plywood is available in thicknesses of (3 ply construction) and upwards; typically aircraft plywood uses veneers of 0.5 mm (approx 1/64 in) thickness although much thinner veneers such as 0.1 mm are also used in construction of some of the thinner panels. Grades Grading rules differ according to the country of