Basically, the tuberculosis bacteria can survive for quite long periods if kept away from ultra-violet light, to which they are sensitive.  Thus they can persist in the air of dark rooms etc.  Particles landing in soil etc do not constitute an infection risk.

To the best of our knowledge, tuberculosis bacilli do not replicate outside the laboratory in natural conditions (i.e. except in specific laboratory culture situations).  Work done in Germany showed that various soil types supported growth of a range of environmental mycobacteria but none supported growth of tubercule bacilli.

There is evidence that health care staff are significantly more likely to be infected by the tubercle bacillus and to develop active tuberculosis than the general public. The actual risk depends on the incidence of tuberculosis in the community and the precise nature of the work of the carer.   Pulmonologists are obviously at risk, but the greatest risk is found among pathologists, especially those performing post-mortem examinations (autopsy, necropsy). Studies among medical students with tuberculosis show that attending necropsies is the greatest risk factor for acquiring the disease.

Historically, pathologists and anatomists were at risk from developing skin tuberculosis following cuts and abrasions acquired during their work - a condition termed prosector's wart.  The famous physician Rene Laenecc, the inventor of the stethoscope, developed such a lesion on his left forefinger following an injury acquired while sawing through a spine of a patient who had died of spinal tuberculosis.

Pathologists can also acquire pulmonary tuberculosis by inhaling tubercle bacilli liberated from infected material while undertaking various examinations, notably while performing necropsies.  The risk varies greatly from region to region depending on the incidence of the disease in the community, and therefore in the bodies submitted to necropsy. 

A particular problem is that tuberculosis may not be diagnosed while the patient is alive, so the pathologist is unwittingly exposed to the risk of infection. In one bizarre incident, a trainee pathologist was performing a necropsy as part of her examination for Fellowship of the Royal College of Pathologists. She diagnosed disseminated tuberculosis, but the diagnosis was disseminated cancer so she failed the examination.  Some months later, the pathologist developed tuberculosis and on re-investigation of the examination material her diagnosis proved to be correct. Needless to say, she sued the College for exposing her to the risk and for failing her!

For people who want to know more about the risk of tuberculosis among pathologists and other laboratory workers you may like to read:  Collins CH, Grange JM. Laboratory- and autopsy-acquired tuberculosis. Communicable Disease and Public Health 1999; 3: 161-167.   This paper also gives information on precautions against infection among laboratory staff, statutory and non-statutory requirements and references to published work. Guidelines on precautions against infection of staff in clinical settings are available from the Centres of Disease Control, Atlanta, GA, USA.

Incidentally, health care professionals other than medical staff are also at increased risk. In a study in the USA, nursing home employees had three times the rate of tuberculosis than other employed adults matched for age, race, and sex.

Chimpanzees can catch TB from humans – in fact they are susceptible to many of the infections humans get.  So if someone had sputum positive (infectious) pulmonary (lung) TB they should not work with chimps (or people!) until they had been on  TB treatment for several weeks and been tested as no longer infectious.

Grade 1 is negative meaning that you do not have TB.  If it had been positive this might have indicated Latent TB (which is not infectious but can turn into active TB if the immune system is compromised – and this would probably warrant preventative treatment just to be sure in a case like yours).   The common symptoms which would indicate active (and possibly infectious) TB disease are a cough, loss of weight, fever and night sweats. If you are working in a country where TB is common, you and your colleagues should watch out for these symptoms and consult a doctor immediately.