To null the impact of tear gas shelling by police, farmers are wearing gas masks as they head to Delhi.
Gas mask, also known as respirator, works as a shield against everything from dust to hazardous gases.
A filter attached to a canister (metal shell) uses a fan to force purified air to the mask.
The constant air flow through the filter results in short life span of the batteries.
It is effective only if used with the correct filter for a particular chemical substance. Selecting the proper filter is a complex process.
There are filters that protect against more than one hazard, but there is no filter that protects against all substances.