And in 2007 a survey of around 200 space companies by the Air Force Research Laboratory cited export controls as the highest barrier to foreign markets.

ECONOMIST: Space technology

In addition, EPA's and USDA's relevant authorities cover a range of natural resources and their supporting ecosystems, including waters, sediments, barrier islands, wetlands, soils, land management, air resources, and drinking water supplies.

WHITEHOUSE: Executive Order -- Gulf Coast Ecosystem Restoration

Preventing this process using an iron-free, corrosion-resistant alloy barrier is particularly important in a city like Boston, where the air is damp and laden with salt water.

ENGADGET: Engineering a better bicycle with DBC City Bike Design Alt

There is also evidence, discovered from the air, that throws into doubt the accepted belief that the wall was a barrier between the empire and the barbarian north.

BBC: Tyne & Wear

To be termed a compound fracture, outside air with bacteria and debris are able to access the fracture site without skin or soft tissue serving as a barrier.

FORBES: Kevin Ware's 'Gruesome' Injury