'Odour Neutralising Agent' (O.N.A.) Solid Block is ideal for placing in internal ventilation systems to eliminate odours in office or household environments or used in small areas such as cupboards and closets or even cars and caravans (for larger space not in a ventilation system, we would recommend the gel). To use, you simply remove the lid or cover and place in the target area. The block works by surface area exposure - the more surface area exposed, the faster and stronger the effect. To regulate this, you can either lessen the effect (if in a small area) by drilling holes in the lid (jar version) and keeping the lid on or for larger ventilation areas, you can take the block completely out of the jar/sachet exposing the entire block. The 'Odour Neutralising Agent' (O.N.A.) Solid Block will release its unique neutralising agent into the atmosphere to treat undesirable odours. Replace when depleted.