I was shocked to discover that my apartment had lead flashing around the roof and balcony, so I researched it a bit, and read some reassuring stuff claiming it was safe (and has less toxic run-off than zinc), and some other less-than-reassuring stuff, so I'm not totally convinced it's safe for my cats to sit in contact with on the balcony.

Of course workers installing it must be careful, but it's claimed to be not harmful to people using the building, or to the environment (from run-off). Just don't lick it or let kids play with it.

On principle, I'd rather have flashing that's ok to lick, and safe for kids to play with and cats to sit on, but that's just not an option at this time.

https://en.wikipedia.org/wiki/Flashing_(weatherproofing)

Not only flashing, but entire roofs can be covered in lead sheeting!
https://nl.pinterest.com/pin/381680137144209527/?lp=true

Lead Flashing: Is it Dangerous?
http://bestqualityroofingandchimney.blogspot.nl/2015/01/lead...

Are lead roofing and flashings a problem?
https://www.fs.fed.us/eng/toolbox/haz/haz22.htm

Lead roofing and flashings are much more likely to be a health hazard for people working with the materials than for people using the building.

Lead has been used for roofing for centuries and is one of the oldest flashing materials. It is durable and soft enough to be formed into complex shapes. Lead roofing and flashings can last over 200 years. Generally speaking, lead roofing or flashing that is in good shape may safely be left in place.

Lead can be used safely if worker exposure is monitored and lead is handled properly to avoid skin contact, ingestion, and exposure to lead dust and fumes.

Lead Sheet Goes Full Circle:
https://elsia.org.uk/sustainability/other-research/

This document sets out to provide a scientifically sound appraisal of the use of lead sheet in construction applications.

Research findings in the document show that lead sheet poses no risk to the environment, even when water run-off is taken into account, and does not present any risk to human beings. Providing simple hygienic precautions are observed people who handle significant quantities of lead do not expose themselves to health risks.

Low environmental impact of lead sheet on buildings

A study conducted in 2005 looked at the environmental impact of lead sheet used in the building industry. With the following conclusions:

There is no risk related to emissions of lead sheet into the environment.
These conclusions are still valid even if it is assumed that all building roofs are disconnected from the sewer system and that the runoff enters the environment directly.

Chemical hazards: Lead:
http://www.health.gov.au/internet/publications/publishing.ns...

Surveys of rainwater from domestic tanks have identified lead contamination as a potential problem. Results have been variable with exceedances of the ADWG health-related guideline value of 10 µg/L (NHMRC & NRMMC 2004) ranging from 0% to 15% of samples collected (SA Health unpublished results 1999-2009; Simmons et al. 2001; Chapman et al. 2006 and 2008; Morrow et al. 2007; Huston et al. 2009; Rodrigo et al. 2009). In addition to these results a survey in Melbourne recorded a particularly high exceedance frequency of 33% (Magyar 2008).

As discussed in other sections the reduction of lead concentrations in paint and the introduction of lead free petrol have decreased the risk from paint and general traffic emissions. However, roof materials and uncoated lead flashing in particular remain a potential source of contamination (Simmons et al. 2001; Chapman et al. 2006; Magyar et al. 2008; Huston et al. 2009), as rainwater tends to be acidic and may mobilise lead, and possibly other metals, into solution. The increasing availability of lead-free or coated lead flashing is expected to reduce future concentrations of lead in rainwater tanks.

Roof catchments used in the collection of rainwater for drinking should not include uncoated lead flashing. Alternative materials should be used in new roofs or when renovating or extending roofs. In existing roofs, painting of uncoated flashing will reduce the risk of contamination. Lead may also enter rainwater from lead washers for roofing screws. These can be replaced with plastic washers.

Leaching of lead into roof run-off may be more of a problem from poorly maintained roofs and gutters, where the process could be increased by the action of water made acidic with organic substances from materials such as leaf litter.